Allow muxer to write exif with APP1

Also contains misc fixes around exif reading/writing.

bug: 123094535
test: verify locally that ExifInterface retrieves the
      exif data at correct offset; HeifWriter accepts
      exif data starting with 'Exif'+APP1 marker.

Change-Id: I526641724caaea68d2b6f1075d9aa30d9a74b973