When Nicki Minaj emerged from a glowing teleporter center stage at Staples Center on Monday night, to the opening hums of her ferocious track “ Roman’s Revenge,” she showed her latest ambition: to be a stadium-packing touring act. It’s the only explanation as to why the rapper, who is hip-hop’s reigning it girl, would return a second time in as many months after supporting Lil Wayne on his I Am Still Music tour.