Annotate Transition#createAnimator with @Nullable

In SystemUI we have a custom implementation of Transition that assumed
that startValues and endValues would never be null. We then started
receiving crash reports and realized that these parameters can be null.

By annotating with @Nullable we can hopefully make it more obvious and
prevent crashes in the future.

Bug: 274002399
Test: N/A
Change-Id: I1b243c7b6d3a6f7912ded35eaf7bf3489598e7f4