Fix ZenModeRepository using the wrong broadcast.

Also optimised the flow a bit behind the flag (doing it in the
background + stateflow instead of sharedflow)

Fix: 347707024
Test: ZenModeRepositoryTest
Flag: com.android.settingslib.flags.volume_panel_broadcast_fix
Flag: android.app.modes_api
Flag: com.android.systemui.new_volume_panel
Change-Id: I97557efc4512196ae703af81622e4a5e1ba89ca5