Restore previous behaviour of hiding pip menu when not focused

- Prior to multi-resume in Q, tapping or starting another activity
  would cause the pip menu activity to pause, which would trigger
  the menu to be hidden, but now, both activities are resumed
  so that logic is not hit (at least through that path). Instead
  the menu can listen for top-resumed-activity changed and use
  that signal for the same behaviour.  This affects the overlay
  test because it uses the trigger to go home to hide the menu
  to ensure there are no sysui task overlays.

Bug: 156314330
Test: atest PinnedStackTests
Change-Id: Ib3b340bc091bb4a94f7b7dc44e08e7af66e4f76f