Toggle wallpaper window flag when setting custom keyguard background

Disable the wallpaper when showing a full-screen custom keyguard
background, e.g. artwork from the RemoteController. This has two nice
effects:

* The wallpaper doesn't visibly "leak" as the keyguard is dismissed
  and fades out

* We save an extra compositing/overdraw step when drawing custom
  backgrounds

Bug 11172944

Change-Id: Id635dcea6b48392f841c9c549327bccff3308c0a