Dump NSSL#isCurrentlyAnimating

Dump whether the NSSL is currently animating or not. This property is
based on the Animator's state, but we've recently started to use new
ones (com.android.internal.dynamicanimation.animation), which we don't
fully trust yet.

While investigating some stale UI states, I'd like to know whether
Animators have completed or not.

Bug: 406222879
Test: dumpsysui NotificationStackScrollLayout
Flag: EXEMPT dumps only
Change-Id: I8b419f82744bdbea3d7347fbcbbf72b249c8e09e