Add api check to guard AnimatorSet behavior change

For pre-N, we have inconsistent behavior between ValueAnimator and
AnimatorSet in the case of calling end() when already ended:
ValueAnimator would start() and immediately end, whereas AnimatorSet
would be no-op. We made a decision to be consistent within Animation
Framework from N forward, which means that AnimatorSet will have the
new behavior of starting and immediately ending just like
ValueAnimator. This new behavior will be guarded by an API check.

Bug: 25601129
Change-Id: I2d952a93d8521c547ec8cde173c80d1d8ead0639