[Flexiglass] Implement brightness mirror

Implement brightness mirror for flexiglass.

In split shade, the NSSL doesn't hide when changing the alpha. Tracking
in separate bug.

Test: manual: portrait, landscape, unfolded, config changes
Test: atest QSSceneAdapterImplTest BrightnessMirrorViewModelTest
Test: atest CentralSurfacesImplTest
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Fixes: 331187804
Bug: 332558331

Change-Id: If5864772cefad02809355f27a35fd4ae84109d55