[bc25] Introduce QuickSettingsShadeOverlay.

This CL only adds the overlay, but does not wire the corresponding user
actions or deprecate the existing QuickSettingsShadeScene which it will
replace.

Bug: 356596436
Flag: com.android.systemui.dual_shade
Flag: com.android.systemui.scene_container
Test: Added QuickSettingsShadeOverlayActionsViewModelTest.
Test: Existing unit tests still pass.
Change-Id: I9450947afe7e034ff03709779ba77458b561845f