Remove root window hierarchy logic from ActivityStackSupervior (40/n)

We eventually want to consolidate root window hierarchy logic into
RootWindowContainer. As a first step towards that we separate
the root window hierarchy logic from ActivityStackSupervior into
a temporary new class RootActivityContainer to make it clear what
we later need to consolidate into RootWindowContainer.

Test: Existing tests pass.
Bug: 80414790
Change-Id: I4e890f14a37e31bf55e7f44dd686d709e1e39b9f