StatusBarDate: Add DesktopInteractor for isDesktopFeatureSetEnabled

Add an interactor to get if desktop feature set is available. This is
used in shade header and will be used in the status bar date.

Bug: 378545144
Flag: com.android.systemui.status_bar_date
Test: ShadeHeaderViewModelTest, DesktopInteractorTest
Change-Id: Iecbe1b805f439ea8a936b33f6cec18cfec9b3ed5