Correct isRunning() behavior for AnimatorSet

isRunning() for an Animator indicates whether an animator has gone
past its start delay and not yet finished. As a subclass of Animator,
AnimatorSet should follow the same principle. The implemention prior
to this CL returns whether any child animation is running, which is
inconsistent with the javadoc for isRunning() and general behavior
for Animator.

Change-Id: Iece814dfcd609ee292dbc00bd55dc64c7bda8e57