3rd step in unifying the window hierarchy that is currently split within AM and WM packages. We separate the the internal interface used to communicate within system server dealing with activities and their containers (tasks, stack, display) from the rest of AM internal interface. Test: Existing tests pass Test: go/wm-smoke-auto Bug: 80414790 Change-Id: Idad77721c1fe10621b9be5dced42a0a11f0183e5