Flag state as contextual filter in views
Not with contextual filter, in views I think it's posible with a normal filter (not exposed) since you have already the flag relationship, first add your flag as a field (don't display it if you want) then just filter your nodes with the flag status (flagged). Then you clone the view and change flag status on filter (now to unflagged)  and you will have an opposite view (non-flagged) content.