Fix AudioFocus for back-to-back notifications

A single instance of NotificationPlayer will play notifications
 back-to-back, and was, for each requesting AudioFocus. But as
 a new notification was about to be played, its accompanying
 thread was terminated, and the completion listener was never
 called, and therefore AudioFocus was not released (e.g. two
 requests for one release).
 The fix consists in requesting AudioFocus only once and
 abandonning it when the only reported completion is received.
 Added a FIXME note about a better solution to be implemented.

Bug 7680947

Change-Id: Ica9d5c1eb5d57c89ceebe66282af69f0f7d43667