Fix AlwaysOnTop stack Z ordering issue

Since in hierarchycal animation, we used WindowContainer#reassignLayer
to compute child layer with cumulative layer number according
specified layer sequence, so we don't use the previous way to get layer
number by WindowContainer#getPrefixOrderIndex.

But in RecentsAnimationController & RemoteAnimationController still uses
the old way to set animation layer, which can no longer use such
conflict way.

Also, make sure WindowContainer#needsZboost as true when the animation
is closing.

Fix: 151024899
Test: atest WindowContainerTest RemoteAnimationControllerTest
      RecentsAnimationControllerTest
Test: manual, launch google map app -> enter PiP mode,
      swipe up to overview screen or launch activity from shortcut,
      make sure the PiP window is not covered by other apps.
Change-Id: I753a8478ca873599d6585a2ec93d1a0fa39e0095