Update focused app when adjusting focus to next stack

Now each displays have their own resumed activities. When a stack
of secondary display is removed and adjust focus to the primary
display, there is no activity state change to update focused app
because the top activity of primary display is already resumed.

Bug: 112119739
Test: atest ActivityManagerMultiDisplayTests#testStackFocusSwitchOnStackEmptied
Change-Id: Id337939149db684cf67ac87bb096c73618e37d72