[flexiglass] Part II: Use single shade on some large-screens.

This is a follow-up to ag/34844807.

Fix: 435145944
Test: Tested manually by rotating an unfolded foldable to portrait and
 observing that single shade is shown.
Test: Added a unit test for this specific case.
Flag: com.android.systemui.scene_container
Change-Id: Id7e5b783f188cae6b1f6ff194661647cd6cd874e