Add layout null check in prepareForBubbleDrag.

Occasionally seeing:
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.android.wm.shell.bubbles.animation.PhysicsAnimationLayout.cancelAnimationsOnView(android.view.View)' on a null object reference at com.android.wm.shell.bubbles.animation.ExpandedAnimationController.prepareForBubbleDrag(ExpandedAnimationController.java:357)

Bug: 419342398
Test: wm presubmit
Flag: com.android.wm.shell.enable_create_any_bubble
Change-Id: Ia69cbe8836b158b6639ded9aac996a369218aa5a