[Flexiglass] Set headsUpTop directly on the NSSL

Instead of looping through a Flow or State, we wan't to set the
headsUpTop value from the composable placeholder to the NSSL, so we have
better control over when does this value reachign the NSSL.

Bug: 339181697
Test: post HUNs, open the shade, open quick settings, check if the value
is set on the NSSL
Flag: com.android.systemui.scene_container

Change-Id: I73afc333de8a15d0cb938bc13c65cacffd425988