Fix NPE for animate frozen snapshot

Before, the snapshot can be null if the window is unfreezed but not
removed from the display changing list, or when it fails to take
screenshot (like because the display is not ready). Now, we make sure to
remove the window from the changing list in such cases.

Fix: 204400505
Test: atest WmTests:WindowContainerTests
Change-Id: I517232fc2ec689680a00b499495a4bad5a0d5f15