[flexiglass] Only update the NSSL's HUN bounds when they're changing

Only make a requesctChildrenUpdate() upon headsUpTop changes, if we
receibe a new value.

Bug: 387961133
Test: drag HUNs, see if they're still updated smoothly
Flag: com.android.systemui.scene_container
Change-Id: I450b06746bd1e1deacd721f600561ab3ad84c601