Fix NPE in ValueAnimator

- Reset mAnimationEndRequested to false in ValueAnimator#start(boolean)
  so that previously canceled animated (with a start delay) can be
  canceled again.
- In ObjectAnimator#animateValue(float) check if target == null
  regardless of mTarget == null since it's possible the target was
  explicitly set to null prior to the animator being cancelled.

Bug: 30190459
Change-Id: I6da55737df30a28f48355fc7a41ab91d05979deb