libstagefright: Populating sync sample index for Video track only.

- While an A/V clip in playback, seek fails to desired location
  and starts playback from zero position.
- The stss atom has zero entry count and sync sample index for Audio
  track. Due to this seek fails and starts playback from zero position.
- Skip populating values from stss atom for Audio track, fix added
  for the same.

Change-Id: I3b9073dc182cf4afc8faf612e1d69493d64d113a
CRs-Fixed: 271385