[HUN] Extract HUN animation y translation values into a shared class.

Both StackScrollAnimator and StackScrollAlgorithm had the same
calculation to determine where a HUN should appear from or disappear to.
This CL extracts that calculation into a shared class.

This will be useful for changing how a HUN animates in and out if the
HUN came from a status bar notification chip tap (see b/393369891).

Bug: 393369891
Bug: 364653005
Flag: com.android.systemui.notifications_hun_shared_animation_values
Test: Verified scenarios in b/243302608#comment39 still work with flag
on
Test: atest StackScrollAlgorithmTest StackStateAnimatorTest
HeadsUpAnimatorTest

Change-Id: Ie4f3703722efd83240e87199f45e06777ffd301b