Add animation when unoccluding windows (1/2)

Before there was a jump-cut when a window that was occluding Keyguard
was going away, leading to an ugly flicker. To fix this, we do the
following.

- Always show windows with FLAG_SHOW_WHEN_LOCKED above lockscreen, even
if they don't "match" the currently occluding app (which is null in the
animation case)
- Move wallpaper behind last window that is not hidden by policy, so the
window doesn't get occluded by the wallpaper.
- Add a flag in the setOccluded call whether to animate or not. SystemUI
then plays a nice animation when it's set.
- Override the animation to always be the animation that happens when we
exit a window which is revealing the wallpaper behind, to make it
consistent with the home screen case.

Fixes: 30829255
Change-Id: Ib3fe20fc9003a0f9f291c974740f044ed8707e75