[Dual Shade] Disable Split Shade in Flexiglass using a config.

Bug: 411365508
Test: Tested manually on a foldable device by disabling Dual Shade and
 observing that Dual Shade is still shown on unfolded state, but Single
 Shade is shown in folded state.
Test: Unit tests still pass.
Flag: com.android.systemui.scene_container
Change-Id: Ibe11629e612c98b052492eeac9996ff2fb551ef7