improve lock screen wallpaper behavior

 - Fix lockscreen wallpaper not being reloaded on user switch.
   There was an issue where the wrong user id was being queried for the
   keyguard wallpaper. Always use the current user id.

 - Also clean up a left-over call for some regular wallpaper
   migration logic that the lock screen wallpaper

 - Add method for removing references to the current lock screen
   wallpaper, which we call from systemui when switching users.

 - Only apply the lock screen wallpaper if there is no media metadata
   to show. There were some cases when it would still appear even when
   music was playing.

Change-Id: I610a38ac11a19638298ca9490b3c87b7ab6106f2
Signed-off-by: Roman Birg <roman@cyngn.com>