DecorView: Avoid changing shadow size in Pinned Stack.

It seems at the end of the animation, sometimes we will
come to think we have focus, before we have properly
updated mStackId. This in-and-of itself seems like
somewhat of an issue...but I'm not inclined to dig too
deeply at the moment. For now just ensure a static shadow
size in the pinned stack.

Bug: 37425446
Test: Manual
Change-Id: I379f7a6de51670c90eacb52a5be5e17784be8ac5