Implement audio focus support in the notification manager.

This change introduces the NotificationPlayer class which was
created from the code of android.media.AsyncPlayer. The only modification
was to modify the construction of the MediaPlayer so it properly issues
onCompletion notifications (which are used to abandon audio focus).

Unless the sound to be played is looped, the notification is transient
and other apps may duck (uses AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK in
audio focus request).
Change-Id: I69cbb71d0892447b934351384e4e24a2e239295b