Fix thumbnail track skipping

Thumbnail tracks are identified by 'tref' box with a 'thmb'
type reference in it. We can't skip a track as soon as a 'tref'
box appears, need to actually parse it and look for 'thmb'.

bug: 77556099

Test: playback of audio files in bug, playback of image sequence
      file (bird_burst.heic) with thumbnail reference.

Change-Id: I7d6ec8af218de2f0cba258f8eaaac9b62f9cb020