libmediaplayerservice:Fix deadlock on gapless start failure

Issue:- Current gapless implementation starts the next clip when
MEDIA_PLAYBACK_COMPLETE event for the first clip is recieved
by MediaPlayerService.
-Even if start for next clip fails, application is notified with
MEDIA_INFO_STARTED_AS_NEXT.
- This causes the framework to trigger mediaplayer start which
again fails as it being for the same clip resulting in a deadlock
in the mediaplayer.
Fix:- Notify application with MEDIA_INFO_STARTED_AS_NEXT only when
start for the next clip is successful.

Change-Id: I77c570074c7c98c996122e275161ba88298e56fc