[Flexiglass] Replaces isSplitShade with shadeMode

As per suggestion from justinweir@, introduces a three-way enum for the
mode of the shade due to upcoming work on dual shade.

Fix: 328815438
Bug: 328473018
Test: unit and integration tests updated
Test: manually verified on foldable that the right shade is shown when
folded and when unfolded
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I7c0c90efb07ca51efe4dc660edbb7a6475780f34