[Flexiglass] Fix ShadeInteractor.isQsFullScreen on split shade

When we are Idle on the QuickSettingsScene, and we are in split shade,
isQsFullScreen should return false, because the QS is NOT taking up
the entire space in this state. This matches the legacy behaviour.

Fixes: 362977962
Test: EmptyShadeView and FooterView shows up correctly in split shade
Flag: com.android.systemui.scene_container
Change-Id: I80164b2642a7e3dfe96506edbd597c9a037009bc