Fix InputConsumer for RecentsAnimationController may access null pointer

Once recents died, it would post a cancel animation
and remove its InputConsumer. So it would be a race condition
when updateInputWindows and remove the InputConsumer.
We need to add a protect for this.

Bug: 120439400
Test: go/wm-smoke
Test: swiping up on the Home button
Change-Id: I5c4323d90ee5a54a2fe655d803be7a3f1561fe2d