Fix Clear all Notifications Animation to avoid QS Panel Flicking

Avoid AmbientState.mScrollY from being set to 0(happens multiple times) during the clear all animation so that QS Panel would not flicking to show up when mScrollY > 0 before clear all.

Bug: 214214514
Test: `atest AmbientStateTest NotificationStackScrollLayoutTest NotificationPanelViewControllerTest` and manual: receive multiple notifications on phone so that there's scroll to show the clear all button. Click clear all, NotificationStackScrollLayout should not scroll to 0 suddenly before shade's closing.
Change-Id: Ie9a0a8aa51278077999873e08cd0c7a0b50f91fb