libstagefright: Add support for files with short audio track and long

video track

- As is, if the audio track is shorter than the video track then
  playback ends when the audio track ends.  This change allows for the
  playback to continue until the video track ends as well.
- The inverse situation with the video track being shorter than the audio
  track is not supported by this change.

Change-Id: I146edb6e1ad1816fe6375d58b86375c88f685b93
CRs-Fixed: 255410