Clean up lock screen hide animation.

We now have an animation to apply to the thing behind the lock
screen animation when it isn't on the wallpaper, which looks
similar to the animation we use when both are on the wallpaper.

In implementing this, cleaned up the code to figure out up-front
which animation to run, getting rid of that kludgy thing that
cleared the window animation if the wallpaper was not being used
for the lower windows.

Change-Id: Ifc4c8a8894ad384124dcf4bbdaab134f1157b0f3