[bc25] Introduce NotificationsShadeOverlay.

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

Bug: 359173565
Flag: com.android.systemui.dual_shade
Flag: com.android.systemui.scene_container
Test: Added NotificationsShadeOverlayActionsViewModelTest.
Test: Existing unit tests still pass.
Change-Id: Icdce2f71c35c7f292f7765cd06f4850213b6cbce