StreamingService: If playing and audio focus is lost, pause the player.

This is really no different than pulling the plug out of the earphone socket,
we pause on that, so we should pause on this as well. The only thing that is
substandard about this is this server might have other listeners. This is the
only sane thing to do in this case, though.
NotificationService: Re-reimplement audio focus.

This time, we'll reimplement a lazy audio focus for the NotificationService
which will try to gain audio focus on notification start but if it's taken
by another music audio service this won't try again until told to update or
start the notification. This behaviour will change slightly in the future
when notification hiding is implemented.