NotificationService: Re-reimplement audio focus.

In this implementation, we implement a lazy and deaf audio focus for
NotificationService. If another music service takes the focus, then we're not
going to request it until next Notification restart or notification button
press, as we don't want to start interrupting local audio playing with remote
MPD users.
CoverManager: Fix local cover art.

This fixes a bug I created in revision 3b43497.