[Dual Shade] Show the notifications shade footer when the shade is empty

BONUS: Update `ShadeTestUtil` to work correctly with Dual Shade as well.

Fix: 404798547
Test: Added unit tests.
Test: Manually by opening the notifications shade with and without
 notifications present, and observing that the footer is shown in both
 cases.
Flag: com.android.systemui.scene_container
Change-Id: I493183bc46b975871ea728f83bd8917017990b40