Mark occluded home stack as invisible.

- The home stack is still visible when a translucent activity (like 
  dialer) is on top, which caused us to use the logic path that just 
  tries to launch the next task.  However, that path does not reload
  the stack state (since the activity stack generally doesn’t change
  while Recents is visible) so it was always launching the already top
  activity.  The new check ensures that we start the activity anew
  as if it was coming from an occluding app.

Bug: 28767764
Change-Id: Iec0fdc0957e5070cec532c5de5cba3454c906a3b