GitLab

Fixed race due to missing mutex around 2 updates to mPlayerPrepared.

MediaPlayerNotificationClient had a raw reference to GenericMediaPlayer;
if for some reason the binder thread out-lived the GenericMediaPlayer,
then it might reference a destroyed object.  Made it a weak reference,
and issue warning if the GenericMediaPlayer has been destroyed.
This has similar effect as a callback protector.

Minor related:
 - mHasVideo is read without mutex, safe since never changed but make it const
 - GenericMediaPlayer's reference to MediaPlayerNotificationClient is now const also

Change-Id: I67b50e861a055c73fd490fb537a3d6d23d25c3d2