Support extracting of thumbnail offsets.

AssetFileDescriptors work by defining a region of an underlying
FileDescriptor across process boundaries, so they require the offset
and length of the EXIF thumbnail region.

Bug: 10412208
Change-Id: I7520fdbbddd7afb1c454bd523ec32eb3602a998f
Merge tag 'tags/android-4.4_r1' into kitkat-staging

Android 4.4 Release 1.0