MMS: Fix Audio file can't play after click play button continuously

when user continuously click the play button, SlideShowActivity
will call onResume()->onPause()->onResume()->onPause() and so
on, so at the last, the MediaPlayer's status is onPause(),

We use a mutex variable to controll it when user continuously
click the play button, we only let play button respond to click
event for one time, if use click more times before the
SlideShowActivity starts, we ignore these events.

CRs-Fixed: 413494

Change-Id: Ie2748c1326672f5ac4e383e66a40edffe826b634