Add flows for events for adding and removing status bar.

These flows utilize CommandQueue.Callbacks#onDisplayRemoveSystemDecorations and #onDisplayAddSystemDecorations

Test: atest com.android.systemui.display.data.repository.DisplayRepositoryTest
Bug: 384694270
Flag: com.android.systemui.shared.status_bar_connected_displays
Change-Id: Ic688ca15f42546632c0557be36fe7b6ae5a57047