Protect code in ViewPropertyAnimator from potential null deref

The monkeys threw a NullPointerException in ViewPropertyAnimator, from
calling into a data structure that should have been not null. Not sure what
weird onkey-driven timing issue caused the problem, but protecting the null
deref is easy and safe.

Issue 5626496

Change-Id: I902b510db60a8a85f776f48582ae0326522a9400