[bc25] Overlays migration: Deprecation of the shade scenes.

This CL deprecates the notifications and quick settings shade scenes,
which are being replaced by the respective overlays.

Entry points (via user actions), scene family resolutions, and
transition definitions will be updated in follow-up CLs. Once those are
done we'll be able to delete the shade scenes.

Summary of additional changes in this CL:
* Simplify the OverlayShade composable by removing the concept of
  backgroundScene and removing its dedicated view-model.
* Add NotificationsShadeSceneContentViewModel.
* Move all rendering of QS content to the QS overlay.
* Clicking on the scrim in legacy shade scenes has no effect anymore.

Bug: 356596436
Flag: com.android.systemui.dual_shade
Flag: com.android.systemui.scene_container
Test: Added unit tests for the new view-model and updated existing ones.
Test: Existing unit tests still pass.
Change-Id: I0c6a9bfb842c18cc33608b20c12af3c4b6bb8726