“Mama Say,” an electro-pop bop from 2017’s “The Valley,” Betty Who pays homage to her idol — and former labelmate — Britney Spears with lines like “Give it to me one more time” and “I’m your slave tonight.”So you can bet the Australian-American singer — who will open for Kesha at Rooftop at Pier 17 on Tuesday (and then the Stone Pony Summer Stage in Asbury Park, NJ on Sept.