Fix a stale animator reference being used on bounds change

The call to setDestinationBounds applies the shadow if animation
type is alpha and it's called on each keep clear area change event,
when sometimes the enter PiP animation has not started yet (hence
there is a stale reference to old animator that should have been
cleaned up in onTaskVanished).

Bug: 275646078
Test: before http://recall/-/ekEuGtt9d9HWqkUtAzpHx8/dXniGevCFIEI5nyLN0zw1V
but only if the .isRuning condition is removed from updateAnimatorBounds
Test: after http://recall/-/ekEuGtt9d9HWqkUtAzpHx8/c3FwTxsPWiRT67JjxcWrMw

Change-Id: I34cb5b7621bf7c087d9c6e5a736b240e01d18a6a