Split internal interface activities from current AM interface (3/n)

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