Fix scroll reset with bundle onboarding UI

When a partially visible notification is removed from the top of
the shade (like during auto-grouping), the scroll position was
being incorrectly reset to an absolute value, causing a jarring
visual jump and potential for the scroll state to become corrupted.

This was most noticeable with Play Store download notifications
and the bundles onboarding UI, but the root cause is in
NSSL#updateScrollStateForRemovedChild

This change corrects the logic to perform a relative scroll
adjustment, preserving visual stability.

Fixes: 427074876
Test: atest SystemUiRoboTests:NotificationStackScrollLayoutTest
Test: see no scroll reset with onboarding UI
Flag: com.android.systemui.notification_bundle_ui

Change-Id: Ie9a020fe4b7212892a24e89a11bf3428f1e1c43c