Add drop shadow for HUNs when shade is opened

Update updateChildZValue to add drop shadows to HUNs when shade is opened, therefore scrim high-priority notifications from its surroundings when the rest part of notification panel slides under it.

Bug: 254935765
Test: StackScrollAlgorithmTest
Manual Test: Check comments under b/254935765
Change-Id: I11a3dec8389e84e079e49e19c6a2bfcac1727d3e
2 files changed