libstagefright: Fix for OMXCodec::read() getting blocked

- Sometimes notification sound is not heard if it is played
  and stopped multiple times in a short duration.
- If OMXCodec::read() gets blocked for filled buffers while
  in pause, it never returns as mFilledBuffers is always empty.
  Due to this AudioTrack destructor never exits as it waits for
  the AudioCallback thread to unblock it.
- Ensure OMXCodec::read() returns even if timeout occurs while
  in pause and handle the same in AudioPlayer by returning zero
  bytes in AudioCallback.
CRs-Fixed: 478768

Change-Id: I295ffa3d7ab836a8f36ae8c10c98c283a4c361d5