Add translateY animation to the removed HUNs animating away

When a HUN is being removed, the StackScrollAlgorithm doesn't find it to
update its translateY value, because it is already removed from its
parent. This CL forces to set the desired translateY value, and makes
sure to start animate to the new ViewState.

Fixes: 323400019
Test: post a HUN, click the action button to remove it, observe the
disapper animation
Flag: ACONFIG notifications_improved_hun_animation DEVELOPMENT

Change-Id: Ie6463dc878bc91c7a8c6c92e32ded92a9f4ae66c