Fix SpringObjectAnimator not sending onAnimationEnd()

We only send onAnimationEnd() if both object animator and spring animator have
ended. However, we also initialized mSpringEnded = false to begin with, which
meant we would never send onAnimationEnd() if the spring animator was never
started. Now we treat the animators as ended until they are started.

Example test:
0: Enable SWIPE_HOME and QUICKSTEP_SPRINGS flags
1: Swipe up and hold on home to go to overview
2: Tap outside overview to go home
3. Swipe up to all apps
4. Swipe down

Expected: animates to home, not overview. Before this change, however, it went
to overview because the state manager never got onAnimationEnd() and therefore
never set the last stable state to NORMAL instead of OVERVIEW.

Change-Id: I7add42eedbc15b3a6a08ac69570da4edd57b5417