Don't show wallpaper when backdrop is visible

Hides the wallpaper when it's not needed and fixes
the unlock animation to not unnecessairly show the
wallpaper if neither the Keyguard nor the underlying
app need it.

Also fixes a bug where the enter animation had a background
set, which led to uglyness when no wallpaper is involved.

Bug: 27533740
Change-Id: I667c6f7ca6c0e1ff7e9f793c6ddc13f6da8387b1