mpris: Fix some typos

Wrong variables are being checked, which prevents the next lines to be
executed.

https://bugzilla.gnome.org/show_bug.cgi?id=729096
albumArtCache: Update thumbnail after lookup

Update the thumbnail property of items so that we do the lookup only
once. This also allows MPRIS to get the URI of the media art.

https://bugzilla.gnome.org/show_bug.cgi?id=729096