[Flexiglass] Add a shadow to HUNs over GONE

HUNs used to have a z translation, when they were displayed outside the
shade over launcher. They have lost it over some refactors, which forces
ENR#isAboveShelf() to return false, which causes NSSL's child placement
logic to hit unexpected paths, which can ultimately result in some
flickering while snoozing a Notification.

Bug: 391204020
Test: post a HUN over launcher -> check its shadow
Flag: com.android.systemui.scene_container
Change-Id: I422207e3bf1c6e6614ec17138ec2d4ccab2ef283