Skip operation on content overlay if task's vanished

The stack trace in b/194344478 and b/194280568 is actually from the
preceding ActivityThreadTest test, which does not fail but does crash
certain test cases after that.

Since we may set a start delay on the content overlay fading out
animation, there is chance that onTaskVanished happens before it is
actually started. It's also possible that onTaskVanished happens during
the content overlay fadeout. This CL should have addressed both cases.

Bug: 194344478
Bug: 194280568
Test: atest --iteration 5 \
      ActivityThreadTest#testHandlePictureInPictureRequested_overriddenToEnter \
      ScreenshotTests#testScreenshotSecureLayers \
      TaskStackChangedListenerTest#testTaskChangeCallBacks
Change-Id: I81c0622966df32e02db7cb5aa3cd42d7c4af735e