Mark TaskSnapshot from onAnimationCanceled Nullable

- TaskSnapshot from onAnimationCanceled can be null, and our Kotlin assumes it's not and throws NullPointerException. Handle it gracefully instead.
- Even though onAnimationCanceled doesn't provide updated screenshot, we still have the screenshot taken on swipe up as backup to show

Fix: 413581605
Test: Manual (see steps in bug)
Flag: EXEMPT bug fix
Change-Id: Ibb4ceb18d9ae17264f5a7addd46801b5762f51cf