Revert "Remove obsolete shadow logic from StackScrollAlgorithm updateChildZValue"

This reverts commit fcb74a62.

Reason for revert: To fix multi-huns bug, because the previous change accidentally removed multi-huns' z-value order.
Bug: 266774171
Test: StackScrollAlgorithmTest

Change-Id: Ie3500b867851647b1647ff4791864573a9c438ae