[flexiglass] Reset stackTop when idle on Gone scene

Because we now derive stackTop from the NotificationPlaceholder composable, when we transition from Lockscreen to Gone (unlocked), stackTop is still incorrectly set to its previous value in the Lockscreen scene. This is visible if we trigger a HUN before ever opening the Shade - it appears at the incorrect, stale stackTop value.

Bug: 357661886
Test: manually verified HUN appears at the correct position after first unlocking the phone and not opening the shade
Flag: com.android.systemui.scene_container
Change-Id: Ib713b4d8fcbbc2672dc56ea085b21676c0cce541