Keyguard: more consistency with the new lockscreen wallpaper

* In the case when the screen was off and setCustomDrawable()
was being called, and the use wallpaper is null, then we could be
setting the color filter on a null drawable.
* Fix logic when old custom background was null and the new one is also
null (but could be the user one) and the func was being returned, thus not
clearing out the current media album art.
* Move resetting the shouldShowWallpaper() flag to the show() method
so it is not being called every time the screen is turned on.
* Fix user image not showing up on first boot if it exists
Change-Id: I567c2ec8920df627835defb822c2ea25be7b3cab
Signed-off-by: Roman Birg <roman@cyngn.com>