Prevents recursive call into end() or cancel()

end() and cancel() in ValueAnimator will trigger onAnimationCancel
or onAnimationEnd callback on its listeners. If additional end()
or cancel() request comes from the callback, a loop will be formed.
Therefore, we need to mark when the end is reuqested so we do not
process end() or cancel() request multiple times during one animation
run, and also effectively terminate the loop.

Bug: 23596652
Change-Id: Iefb69eb8969071b43124c09d7cccbe9ff5ba5dcc