[Flexiglass] Follow HunSharedAnimationValues refactor in SSA

The NotificationHunSharedAnimationValues refactor has stopped using
AmbientState#mHeadsUpBottom. This CL updates the last path that was
reading this field.

Bug: 400691495
Flag: com.android.systemui.notifications_hun_shared_animation_values
Flag: com.android.systemui.scene_container
Test: StackScrollAlgorithmTest
Test: check if we don't hit the new legacy assertions in AmbientState
Change-Id: I290b303c5b3c3ab767f33a896d4b53eb27a428fe