Partially reverts ag/32760467 (reparent instead of relative layer).

We decided to revert to this approach because the problems with
reparenting are more feasible to fix than those with the relative
layer, which revolve around the new AppZoomOut.

rgl@ also thinks this is just a better approach in general and I
believe him.

Bug: 404508609
Fix: 407021260
Flag: com.android.systemui.move_transition_animation_layer
Test: atest ActivityTransitionAnimatorTest
Change-Id: I56bf1bdd447abcf7805da230d1fa21dee559031f