预备知识：External Storage Technical Information
摘要：
"The WRITE_EXTERNAL_STORAGE permission must only grant write access to the primary external storage on a device. Apps must not be allowed to write to secondary external storage devices, except in their package-specific directories as allowed by synthesized permissions. Restricting writes in this way ensures the system can clean up files when applications are uninstalled."
"Starting in Android 4.4, multiple external storage devices are surfaced to developers through Context.getExternalFilesDirs(), Context.getExternalCacheDirs(), and Context.getObbDirs()."
Q&A：
*Primary external storage? Secondary external storage?
-参考source.android.com/devices/tech/storage/config-example.html
有的手机不支持SD卡扩展（如Nexus 4），所以没有secondary；
有的手机支持（如Note 3），所以区分primary和secondary。
是否primary，由android/frameworks/base/core/res/res/xml/storage_list.xml决定。
一般地，primary是由内部存储（internal storage，比如NAND eMMC）中划分一部分为外部存储（external
storage）；mountPoint为"/storage/sdcard0"；subsystem为"fuse"，也就是由FUSE
daemon模拟的。
secondary是可插拔（removable）的SD卡，mountPoint为"/storage/sdcard1"或"/storage/extSdCard"；subsystem为"sd"。
*什么是package-specific directories？
-如package名为com.app.foo在primary/secondary的Android/data/com.app.foo目录。
/Android/data目录是由android/system/core/sdcard.c::handle_mkdir被调用时创建的，并同时新建.nomedia文件。
Context.getExternalFilesDirs(), Context.getExternalCacheDirs()会触发创建package-specific directories。
而Context.getObbDirs()会触发创建/Android/obb/。
*app对于SD卡的write权限范围分别在哪儿？
-在Note3上试验:
1.不声明WRITE_EXTERNAL_STORAGE情况下，不管是primary还是secondary，在package目录下能够写文件，在Android/data/下都会抛出IOException；
2.声明后，primary能够在Android/data/目录下写文件，而secondary依然IOException。
"For
example, the app with package name com.example.foo can now freely
access Android/data/com.example.foo/ on external storage devices with no
permissions."
"Specifically, configuration and log files should only be stored on internal storage where they can be effectively protected."