Unify Activities and Windows under one lock (38/n)

Final step before we actually start unifing the hierarchy is to have
activity and window manager under one lock. This CL makes it so.
Note, that there are probably areas where this change might lead to dealock
however we can fix those areas as we discover them.

Test: Existing tests pass
Bug: 80414790
Change-Id: Ie4b71d4ceb986256482031a99065b8d2ef60ebe8