Fix cancellation of AnimatorSet when child animation has delay

Previously, AnimatorSet incorrectly checked whether child animations were
'running' to figure out what to cancel. If a child animation was started, but
sitting in a startDelay phase, it was not 'running', so the right cancel/end
events would not propagate.

The fix is to add a new isStarted() API to Animator, which returns true when
the animator has started (but not yet ended), regardless of whether the animator
has a startDelay or not. It's basically a superset of the existing isRunning()
method, which only returns true when an animator has actually started setting values.

Change-Id: I126814cb6637b58295b6d18d9b155235671f99be