[bc25] Don't render lockscreen notifications on NotificationsShadeScene.

The notifications shade already renders its own notifications stack, so
we should avoid rendering lockscreen notifications in that scene.

This prevents unnecessarily rendering notifications twice, as well as
composing the same element key twice in the same scene.

In a future CL I'll make sure notifications shade renders its
notifications in the same position, making this imperceptible for the
user.

Flag: com.android.systemui.dual_shade
Flag: com.android.systemui.scene_container
Bug: 340553529
Test: Manually verified by opening and closing the notifications shade
on the lockscreen and observing that lockscreen notifications disappear.
Test: Added unit tests, and existing unit tests still pass.

Change-Id: Iba15e6c8bf71ef6d8f29b1b3d3e46b7512a6d6f2