Moved Activity management out of ActivityManager to WindowManager (37/n)

What could go wrong ;)

Note that ATMS and all other activity management files are still under
the AMS lock. We will be changing them soon to use the WMS lock, but
till then it is important to take care not to use the WMS lock for
activities stuff and AMS lock for window stuff.

Test: Existing tests pass
Bug: 80414790
Change-Id: I69f5a6e40ad892557a425e1f34be987507102136