Fix VOLUME_PANEL_SHOWN event

VOLUME_PANEL_SHOWN is accidentally removed in ag/26833809, it affects the dashboard data in recent month.

Flag: com.android.systemui.new_volume_panel
Test: verified the event is triggered when opening volume panel
Bug: 322279959
Change-Id: I2a8f89d9c4433bc016fb16528ffe87b7c7f7a7b2