Introduce DreamScene

This change adds a dedicated scene for dreams. The scene will eventually
show when a dream activity is present.

This scene renders a sleep emoji for now to make it visually identifiable.

Test: (with a local change) able to transition to the dream scene
Bug: 368599130
Flag: com.android.systemui.scene_container
Change-Id: I314bdecd26617ccaeb62f0514c1946ef5d5320e6