Fix bug 2605504 Don't leak a metric ton of NotificationPlayer threads

The Looper on the thread created in order to be notified of the
playback completion of notification sounds, was never stopped, causing
the threads to stay around in wait state.
The fix consists in quitting the Looper used for the previous sound
when a new sound is started.

Change-Id: Ia4b3d16b5447ffafacc093db3a7fe05574089935