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:7eb6b3fa1557873423d2fc6c286103ef141a24ff)
Merged-In: Ic4aaa8e1a50a4e2a8b7b87baff5f2b1819a7e8ab
Change-Id: Ic4aaa8e1a50a4e2a8b7b87baff5f2b1819a7e8ab
1 file changed