Fix regression when removing task.

- Limit removing the task immediately to when there is a single task
  overlay activity. Certain activities may not work when force-removed
  before the clean up work in removeTask() completes especially if the
  finishing activity is a part of a launch process (ie. trampoline
  activity). This behaviour is only needed for finishing activities that
  have actual task overlays.

Bug: 36761629
Test: Launch SiriusXM, and check that it loads
Change-Id: I34c812ab111819e858fe36b6359f82da717ba459