Move the wallpaper beneath the keyguard.

If the keyguard is the wallpaper target the wallpaper cannot sit at
the bottom of the stack and must be directly beneath the keyguard.
Otherwise keep it at the bottom of the window stack.

App animations when the keyguard is showing should not be disabled if
the keyguard is also animating.

Fixes bug 10858941.
Fixes bug 10932680.

Change-Id: I8399837f6510ea16003f68b165e67439f3571ef4