Fix quick switching to any task that already appeared

Previously we did this for the last task that appeared, but didn't
take into account cases where multiple tasks had appeared during
the same gesture state. We handle that now.

Test:
- Quick switch to delayed task A, start new gesture, return to the
  first app after onTaskAppeared(A)
- Quick switch to delayed task A, start new gesture, wait for
  onTaskAppeared(A), then switch to delayed task B, start new
  gesture, wait for onTaskAppeared(B), and switch back to A
- Quick switch to delayed task A, start new gesture, wait for
  onTaskAppeared(A), then switch to delayed task B, start new
  gesture, wait for onTaskAppeared(B), and switch back to A and
  quickly back to original app (before settling on A)
Ensure task starts/resumes and is interactable in all cases.

Bug: 158588954
Change-Id: I647a6d015baa1d9b6d613e6c0e584589f35da54a