Use the top stack on display instead of focused

There are places where there is an assumption that there is only
one display with activities in the system and that the focused stack
is usually the topmost stack on current display.
Here we explicitly use the topmost from the stacks on a display
instead of focused stack record in ActivityStackSupervisor.

Bug: 34263289
Bug: 35258038
Test: android.server.cts.ActivityManagerDisplayTests
Test: #testStackFocusSwitchOnDisplayRemoved
Test: #testStackFocusSwitchOnDisplayRemoved2
Change-Id: Ice1a555b0a4460d6bd63be2af7b51b145c85ab7f