libstagefright: LPA: Fix for delay in stopping LPA playback

- Start MP3/AAC clip with duration > 1min.
  Go to settings --> apps --> Running --> Music and stop
  MediaplaybackService. The audio playback does not stop
  immediately but continues for next couple of seconds.
- While trying to tear down the LPA playback path, if the
  decoder thread is blocked on write to audio HAL, it is
  unable to stop audio playback until the decode thread
  gets unblocked.
- Fixed by issuing flush() before killing the decoder thread
  while LPA playback tear down.

Change-Id: I9d7a0c583f870db243c76fe4f38e2ae751176ab8
CRs-Fixed: 399499