libstagefright: stop audio source before deleting AudioPlayer

-audio source won't be stopped during AudioPlayer destruction
 due to start failed and AUDIOPLAYER_STARTED not set
-handle audio source stop in AwesomePlayer if AUDIOPLAYER_STARTED
 not set

Change-Id: I395c045a0a8a5d5b6ea8cc762d6a2636bb885b0e
CRs-Fixed: 704605