Check if displays allow adding system decorations instead of just relying on...

Check if displays allow adding system decorations instead of just relying on onDisplayAddSystemDecorations callbacks

Before the CD project, aka when content mode flag is OFF, onDisplayAddSystemDecorations() is always sent to status bar upon a new display is created.

Test: atest com.android.systemui.display.data.repository.DisplayRepositoryTest
Bug: 424787145
Flag: com.android.systemui.shared.status_bar_connected_displays
Change-Id: I8078fab2a92cf62e436cdb98b547dd7db0483091