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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:eddc3f6525f554790dc4b7f849da2ab12e6e116e)
Merged-In: Ibb4ceb18d9ae17264f5a7addd46801b5762f51cf
Change-Id: Ibb4ceb18d9ae17264f5a7addd46801b5762f51cf