错误条件
按照《第一行代码》中操作，给app模块添加依赖：compile 'com.android.support.percent:24.2.1'，gradle同步的时候报出以下错误：
ERROR: Failed to resolve: com.android.support.percent:24.2.1:
Affected Modules: app
WARNING: Configuration 'compile' is obsolete and has been replaced with 'implementation' and 'api'
It will be removed at the end of 2018. For more information see: http://d.android.com/r/tools/update-dependency-configurations.html
Affected Modules: app
解决过程
后上网查阅，发现两个错误：
- 我的Android Studio的版本是3.4.1（可以通过菜单栏Help-About查看），Android studio升级到3.0之后，compile就改为了implementation
- SDK版本是29（在app模块的build.gradle中指明)，依赖的版本24.2.1 应该改为和sdk版本对应的版本29.0.0
于是就将百分比布局的依赖改成了：implementation 'com.android.support.percent:29.0.0', 再次同步，依然报错：
ERROR: Failed to resolve: com.android.support.percent:24.2.1:
Affected Modules: app
再次百度，发现了一件事：Androidx库和Android.support库不能共存，而我其它的依赖都用的是Androidx库，如下：
implementation 'androidx.appcompat:appcompat:1.0.2'
implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
那么我的问题就转化成了：找到percentLayout依赖在androidx库中的写法，得到的结果是androidx.percentlayout:percentlayout:1.0.0，再次同步，成功！
最终解决办法
依赖的写法改为：implementation 'androidx.percentlayout:percentlayout:1.0.0'
以下附上Androidx的映射表，方便以后查看
android.arch.core:common
androidx.arch.core:core-common:2.0.0-rc01
android.arch.core:core
androidx.arch.core:core:2.0.0-rc01
android.arch.core:core-testing
androidx.arch.core:core-testing:2.0.0-rc01
android.arch.core:runtime
androidx.arch.core:core-runtime:2.0.0-rc01
android.arch.lifecycle:common
androidx.lifecycle:lifecycle-common:2.0.0-rc01
android.arch.lifecycle:common-java8
androidx.lifecycle:lifecycle-common-java8:2.0.0-rc01
android.arch.lifecycle:compiler
androidx.lifecycle:lifecycle-compiler:2.0.0-rc01
android.arch.lifecycle:extensions
androidx.lifecycle:lifecycle-extensions:2.0.0-rc01
android.arch.lifecycle:livedata
androidx.lifecycle:lifecycle-livedata:2.0.0-rc01
android.arch.lifecycle:livedata-core
androidx.lifecycle:lifecycle-livedata-core:2.0.0-rc01
android.arch.lifecycle:reactivestreams
androidx.lifecycle:lifecycle-reactivestreams:2.0.0-rc01
android.arch.lifecycle:runtime
androidx.lifecycle:lifecycle-runtime:2.0.0-rc01
android.arch.lifecycle:viewmodel
androidx.lifecycle:lifecycle-viewmodel:2.0.0-rc01
android.arch.paging:common
androidx.paging:paging-common:2.0.0-rc01
android.arch.paging:runtime
androidx.paging:paging-runtime:2.0.0-rc01
android.arch.paging:rxjava2
androidx.paging:paging-rxjava2:2.0.0-rc01
android.arch.persistence.room:common
androidx.room:room-common:2.0.0-rc01
android.arch.persistence.room:compiler
androidx.room:room-compiler:2.0.0-rc01
android.arch.persistence.room:guava
androidx.room:room-guava:2.0.0-rc01
android.arch.persistence.room:migration
androidx.room:room-migration:2.0.0-rc01
android.arch.persistence.room:runtime
androidx.room:room-runtime:2.0.0-rc01
android.arch.persistence.room:rxjava2
androidx.room:room-rxjava2:2.0.0-rc01
android.arch.persistence.room:testing
androidx.room:room-testing:2.0.0-rc01
android.arch.persistence:db
androidx.sqlite:sqlite:2.0.0-rc01
android.arch.persistence:db-framework
androidx.sqlite:sqlite-framework:2.0.0-rc01
com.android.support.constraint:constraint-layout
androidx.constraintlayout:constraintlayout:1.1.2
com.android.support.constraint:constraint-layout-solver
androidx.constraintlayout:constraintlayout-solver:1.1.2
com.android.support.test.espresso.idling:idling-concurrent
androidx.test.espresso.idling:idling-concurrent:3.1.0
com.android.support.test.espresso.idling:idling-net
androidx.test.espresso.idling:idling-net:3.1.0
com.android.support.test.espresso:espresso-accessibility
androidx.test.espresso:espresso-accessibility:3.1.0
com.android.support.test.espresso:espresso-contrib
androidx.test.espresso:espresso-contrib:3.1.0
com.android.support.test.espresso:espresso-core
androidx.test.espresso:espresso-core:3.1.0
com.android.support.test.espresso:espresso-idling-resource
androidx.test.espresso:espresso-idling-resource:3.1.0
com.android.support.test.espresso:espresso-intents
androidx.test.espresso:espresso-intents:3.1.0
com.android.support.test.espresso:espresso-remote
androidx.test.espresso:espresso-remote:3.1.0
com.android.support.test.espresso:espresso-web
androidx.test.espresso:espresso-web:3.1.0
com.android.support.test.janktesthelper:janktesthelper
androidx.test.jank:janktesthelper:1.0.1
com.android.support.test.services:test-services
androidx.test:test-services:1.1.0
com.android.support.test.uiautomator:uiautomator
androidx.test.uiautomator:uiautomator:2.2.0
com.android.support.test:monitor
androidx.test:monitor:1.1.0
com.android.support.test:orchestrator
androidx.test:orchestrator:1.1.0
com.android.support.test:rules
androidx.test:rules:1.1.0
com.android.support.test:runner
androidx.test:runner:1.1.0
com.android.support:animated-vector-drawable
androidx.vectordrawable:vectordrawable-animated:1.0.0
com.android.support:appcompat-v7
androidx.appcompat:appcompat:1.0.0
com.android.support:asynclayoutinflater
androidx.asynclayoutinflater:asynclayoutinflater:1.0.0
com.android.support:car
androidx.car:car:1.0.0-alpha5
com.android.support:cardview-v7
androidx.cardview:cardview:1.0.0
com.android.support:collections
androidx.collection:collection:1.0.0
com.android.support:coordinatorlayout
androidx.coordinatorlayout:coordinatorlayout:1.0.0
com.android.support:cursoradapter
androidx.cursoradapter:cursoradapter:1.0.0
com.android.support:customtabs
androidx.browser:browser:1.0.0
com.android.support:customview
androidx.customview:customview:1.0.0
com.android.support:design
com.google.android.material:material:1.0.0-rc01
com.android.support:documentfile
androidx.documentfile:documentfile:1.0.0
com.android.support:drawerlayout
androidx.drawerlayout:drawerlayout:1.0.0
com.android.support:exifinterface
androidx.exifinterface:exifinterface:1.0.0
com.android.support:gridlayout-v7
androidx.gridlayout:gridlayout:1.0.0
com.android.support:heifwriter
androidx.heifwriter:heifwriter:1.0.0
com.android.support:interpolator
androidx.interpolator:interpolator:1.0.0
com.android.support:leanback-v17
androidx.leanback:leanback:1.0.0
com.android.support:loader
androidx.loader:loader:1.0.0
com.android.support:localbroadcastmanager
androidx.localbroadcastmanager:localbroadcastmanager:1.0.0
com.android.support:media2
androidx.media2:media2:1.0.0-alpha03
com.android.support:media2-exoplayer
androidx.media2:media2-exoplayer:1.0.0-alpha01
com.android.support:mediarouter-v7
androidx.mediarouter:mediarouter:1.0.0
com.android.support:multidex
androidx.multidex:multidex:2.0.0
com.android.support:multidex-instrumentation
androidx.multidex:multidex-instrumentation:2.0.0
com.android.support:palette-v7
androidx.palette:palette:1.0.0
com.android.support:percent
androidx.percentlayout:percentlayout:1.0.0
com.android.support:preference-leanback-v17
androidx.leanback:leanback-preference:1.0.0
com.android.support:preference-v14
androidx.legacy:legacy-preference-v14:1.0.0
com.android.support:preference-v7
androidx.preference:preference:1.0.0
com.android.support:print
androidx.print:print:1.0.0
com.android.support:recommendation
androidx.recommendation:recommendation:1.0.0
com.android.support:recyclerview-selection
androidx.recyclerview:recyclerview-selection:1.0.0
com.android.support:recyclerview-v7
androidx.recyclerview:recyclerview:1.0.0
com.android.support:slices-builders
androidx.slice:slice-builders:1.0.0
com.android.support:slices-core
androidx.slice:slice-core:1.0.0
com.android.support:slices-view
androidx.slice:slice-view:1.0.0
com.android.support:slidingpanelayout
androidx.slidingpanelayout:slidingpanelayout:1.0.0
com.android.support:support-annotations
androidx.annotation:annotation:1.0.0
com.android.support:support-compat
androidx.core:core:1.0.0
com.android.support:support-content
androidx.contentpager:contentpager:1.0.0
com.android.support:support-core-ui
androidx.legacy:legacy-support-core-ui:1.0.0
com.android.support:support-core-utils
androidx.legacy:legacy-support-core-utils:1.0.0
com.android.support:support-dynamic-animation
androidx.dynamicanimation:dynamicanimation:1.0.0
com.android.support:support-emoji
androidx.emoji:emoji:1.0.0
com.android.support:support-emoji-appcompat
androidx.emoji:emoji-appcompat:1.0.0
com.android.support:support-emoji-bundled
androidx.emoji:emoji-bundled:1.0.0
com.android.support:support-fragment
androidx.fragment:fragment:1.0.0
com.android.support:support-media-compat
androidx.media:media:1.0.0
com.android.support:support-tv-provider
androidx.tvprovider:tvprovider:1.0.0
com.android.support:support-v13
androidx.legacy:legacy-support-v13:1.0.0
com.android.support:support-v4
androidx.legacy:legacy-support-v4:1.0.0
com.android.support:support-vector-drawable
androidx.vectordrawable:vectordrawable:1.0.0
com.android.support:swiperefreshlayout
androidx.swiperefreshlayout:swiperefreshlayout:1.0.0
com.android.support:textclassifier
androidx.textclassifier:textclassifier:1.0.0
com.android.support:transition
androidx.transition:transition:1.0.0
com.android.support:versionedparcelable
androidx.versionedparcelable:versionedparcelable:1.0.0
com.android.support:viewpager
androidx.viewpager:viewpager:1.0.0
com.android.support:wear
androidx.wear:wear:1.0.0
com.android.support:webkit
androidx.webkit:webkit:1.0.0
参考文献请点击：这里