Cancel veil animations on dispose

The veil animations may be ongoing when a window decoration is destroyed
and thus its veil surface is released, which leads to an NPE since the
surface has been removed and made null by the time the animator tries to
update it. This change cancels any animations before the surface is
released to prevent the crash.

Bug: 322570124
Test: drag a freeform task to split-left, open another app to
split-right, drag the left task from the handle into the split-left
visual indicator, verify no crash.

Change-Id: Ibce242104aa6fa46b57ed3b1fe36128ae34c97bf