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

core/java/android/animation/ObjectAnimator.java

@@ -977,7 +977,7 @@ public final class ObjectAnimator extends ValueAnimator {
     @Override
     void animateValue(float fraction) {
         final Object target = getTarget();
-        if (mTarget != null && target == null) {
+        if (target == null) {
             // We lost the target reference, cancel and clean up.
             cancel();
             return;

core/java/android/animation/ValueAnimator.java

@@ -982,6 +982,7 @@ public class ValueAnimator extends Animator implements AnimationHandler.Animatio
         mStarted = true;
         mPaused = false;
         mRunning = false;
+        mAnimationEndRequested = false;
         // Resets mLastFrameTime when start() is called, so that if the animation was running,
         // calling start() would put the animation in the
         // started-but-not-yet-reached-the-first-frame phase.