libstagefright: Fix for data loss in qcp recording

- For evrc and qcelp recording, nearly 1 sec of data which is
  recorded is not writtern to the recorded clip.
- The writer thread which writes the encoded data to .qcp file,
  is not getting enough CPU time. This results in ~500ms of data
  loss in the recorded clip.
- Fix is to increase the priority of the writer thread to
  ANDROID_PRIORITY_AUDIO

Change-Id: I19125593ade2697251df976c81cb4df8dcf7b80b
CRs-fixed: 386398