Disallow onExitPipFinished() from being called multiple times

- We really should fix the root cause, but return early for now to
  unblock shell transitions
- Ensure tests call onTaskAppeared() with a valid surface control

Bug: 239461594
Test: atest WMShellFlickerTests
Change-Id: I9ca41537ab6128ffde9d2061be3e2bc16d0a3f0b