Remove lingering content overlays in onTaskVanished

onTaskVanished already attempted to do some last resort overlay
cleanup, but because onExitPipFinished was called right above it, and
it internally set the state=UNDEFINED, the overlay removal in
onTaskVanished was always a no-op.
This change changes the order of onExitPipFinished and the animator
cancelling/overlay removal so that there is always an attempt to do
the removal in on vanish.
There should be no risk with this change that the removal is done twice
since even if the overlay is removed by an animator in onAnimationEnd,
the removal in onTaskVanished first checks that an animator exists and
that a content overlay exists, and one or both of those should cause
it to no-op if someone else removed the content overlay earlier.

Bug: 230561935
Test: enter MS teams call, swipe edge to go back, verify there's no
fullscreen overlay lingering.
Test: atest --iteration 5 \
      ActivityThreadTest#testHandlePictureInPictureRequested_overriddenToEnter \
      ScreenshotTests#testScreenshotSecureLayers \
      TaskStackChangedListenerTest#testTaskChangeCallBacks

Change-Id: If8639ce78148c5ba1b247bb8afeee76e20863dc5