audio: handle events properly in AudioOutput::CallbackWrapper()

- MediaPlayerService::AudioOutput::CallbackWrapper() can't handle
  AudioTrack::EVENT_STREAM_END and AudioTrack::EVENT_NEW_IAUDIOTRACK

- Fix is to handle the events separately.

Change-Id: I986ce8d859dced19fa0bc82acb180fbd11417ca9
CRs-Fixed: 678573