Keep the pulsing notifications in place while animating away

Keep the pulsing HUNs at their original translateY animation while
animating away.

Bug: 325939444
Test: atest StackScrollAlgorithmTest
Test: Post alerting notifications on the AOD, see how they animate away.
Flag: ACONFIG com.android.systemui.notifications_improved_hun_animation TEAMFOOD

Change-Id: Ifb36db455cc65eaa465319b91daaf7d9dfaa00c1