[flexiglass] Fix HUN placeholder remaining visible after the Shade is opened and closed

In legacy, opening the shade calls HeadsUpManagerPhone.onExpandingFinished (the call comes from NPVC). This leads to the HUN entry being removed, which in flexiglass will trigger the HUN placeholder disappearing. We need to relocate this side-effect to depend on scene state instead, in order to avoid leaving the placeholder and flexiglass visibility in a bad state.

Bug: 357661886
Test: manually verified that notification placeholder and flexiglass are not visible after receiving a HUN, opening shade, and then closing shade.
Test: manually verified with logging that HeadsUpManager entries are updated correctly upon changing scenes.
Flag: com.android.systemui.scene_container
Change-Id: Id78e47f9b384cbf5ffc42c8845d893043d2a0615