Fix bug 2599698 Music paused by itself

The NotificationPlayer sets an OnCompletionListener on the
MediaPlayer it's using to play the notification.
NotificationPlayer relies on the completion event to abandon
audio focus.
 The player was started before the listener was set. The theory
about the bug is that the player finished playing before the listener
was called. Therefore the audio focus was never abandonned, and
the music didn't resume.

Change-Id: Ic3baf359ea24de0d832a655dc47e745b25bc3dec