GitLab

The StreamSource callback thread was continuing to run after the
MediaPlayer object was destroyed.  Fixed by adding a callback protector
and a pre-destroy hook. GenericMediaPlayer::preDestroy now also
calls MediaPlayer::stop just in case.

Change-Id: I5bd771d4d1936f433d2a8c9959593782c96daed9
Miscellaneous:
 - added an explicit destructor on CMediaPlayer::mAVPlayer for consistency
   (probably not a bug since there was a clear on it earlier)
 - updated comments for CallbackProtector
 - made some CallbackProtector fields private since no sub-classes yet