[flexiglass] Use single shade on some portrait-oriented large-screens.

Currently, we use Split Shade on tablets and certain unfolded foldables
in portrait orientation. In legacy (pre-Flexiglass) shade, Single Shade
was used instead. This CL is matching the legacy behavior.

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: I5b5a2d87ea5b19b9ba2516f062e4f7ab5843ff8b