Return EXIF thumbnails when available.

Adds new ExifInterface method to extract the thumbnail range from
a larger image file, and use that to return an AssetFileDescriptor.

When decoding an AssetFileDescriptor thumbnail with offsets, read out
the raw data entirely, since Skia uses lseek() aggressively.

Bug: 10412208
Change-Id: I7906cdf82c0c3794cec7043c801a86f66efeb143