[bc25] Create the `NotificationsShade` scene.

The scope of the CL includes the following:

* Disable the legacy shade scene when Dual Shade is enabled.
* New scene key for the Notification Shade scene.
* New module for the Notification Shade scene.
* Conditional references to the new scene key wherever appropriate.
* Hydrate the ShadeMode based on the dual shade flag status.
* Define the scene navigational mapping.
* Create the new scene composable, reusing OverlayShade.
* Create the scene view model with basic destination scenes mapping.
* New unit tests for the NotificationsShadeSceneViewModel.

Test: Manually verified by swiping down and swiping up on both the
 lockscreen and gone scenes, with the feature flag both enabled and
 disabled.
Test: Added unit tests.
Fix: 338009491.
Flag: ACONFIG com.android.systemui.dual_shade DEVELOPMENT
Change-Id: Ifce3270eb1e92912d59fdccdb635b1fe28a15925