Keyguard : Fix album art sticking after playback is stopped

newIsNull should be populated by what is currently being set and not by
what the user's custom background is, since otherwise you end up a in state
where the user has no custom background set, music stops, no album art via arg is provided and you simply return.

Change-Id: Ifaea1cae176faaba6b32a711071f3b75ccacdd63