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.

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.
StreamingService: windDown after error.

Right now trying to stream again after an error is causing more problems
than it's fixing. Disable and perfect later.
Merge pull request #518 from avuton/next

Fixes