Reset the preferred top focus stack when task visibility changes

- The change in visibility of a task does not always trigger a
  stack order change, which won't trigger the previously set
  top focus stack to be reset for the next activity launch.

Bug: 154658401
Test: Verify we don't regress on b/112084174, and open an activity
      from a bubbled task
Change-Id: I8770448b3ac40c384e8e93ca7ee37f1f8aa80cf4