Fix selecting focused stack with secondary displays

When the last activity finishes in the stack we're looking for other
stacks and making it focused. However we weren't doing that if the
stack was on a secondary display, so the focused stack records were
not updated in stack supervisor.

Now we're looking for other stacks on the same display first. If there
is nothing focusable left - shifting focus to next focusable display.

Test: android.server.cts.ActivityManagerDisplayTests
Test: #testStackFocusSwitchOnDisplayRemoved
Test: #testStackFocusSwitchOnStackEmptied
Change-Id: Ifbb893e12cbe9c4928b949a86fc8bc027de181e4