Animate addition/removal of views in expanded mode.

This required adding the setChildVisibility method to controllers, to allow them to animate in/out views that pass the max rendered child threshold. This was not previously relevant since in the bubble stack, you can't really see the views when they're set to VISIBLE/GONE.

Also, renamed onChildToBeRemoved to onChildRemoved since that's more accurate given the move to transient views.

Test: atest SystemUITests
Change-Id: I291ff8f6257ba54e0688c1062bbd673e0c7bdb5c