Fix nonhiding album art

* The album art was not being hidden if the URI for it did not exist and as a result, this could lead to mismatched songs and album art if the second-infinity song in the playlist did not have album art.
* Also cleaned up the way the now playing text is displayed.  It is set to "" in the function that gets called, and as a result can just be set at the beginning of the function.

Change-Id: I8e5670403b4ad63a212f148d95038597c31625ec