提供了四个对外的静态接口
1. Bitmap android.media.ThumbnailUtils.createImageThumbnail(String filePath, int kind)
This method first examines if the thumbnail embedded in EXIF is bigger than our target size. If not, then it'll create a thumbnail from original image. Due to efficiency consideration, we want to let MediaThumbRequest avoid calling this method twice for both kinds, so it only requests for MICRO_KIND and set saveImage to true. This method always returns a "square thumbnail" for MICRO_KIND thumbnail.
Parameters:
filePath the path of image file
kind could be MINI_KIND or MICRO_KIND
Returns:
Bitmap
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This method is only used by media framework and media provider internally.
2. Bitmap android.media.ThumbnailUtils.createVideoThumbnail(String filePath, int kind)
返回视频缩略图，如果视频损坏或格式不被支持那么都将返回为空
filePath：视频文件的位置
kind：Images.Thumbnails.MICRO_KIND或者Images.Thumbnails.MINI_KIND最终和分辨率有关
3. Bitmap android.media.ThumbnailUtils.extractThumbnail(Bitmap source, int width, int height)
创建一个指定大小居中的缩略图
4. Bitmap android.media.ThumbnailUtils.extractThumbnail(Bitmap source, int width, int height, int options)
创建一个指定大小居中的缩略图，如果options定义为OPTIONS_RECYCLE_INPUT，则回收资源
ThumbnailUtils类是API Level从8或更高才开始支持的，不向下兼容