SARAH MILLS IMAGINE a time more than 50 years ago when four of the biggest Black American celebrities in the world met up and spent hours discussing race, religion and politics? That's the premise of the film "One Night in Miami," out on Amazon Prime on Friday, which recounts a 1964 meeting between boxer Cassius Clay, soul singer Sam Cooke, civil rights activist Malcolm X and football player Jim Brown. The meeting really took place, says screenwriter Kemp Powers, who came across a throwaway mention of it years ago and became obsessed with finding out more. "The characterizations and the…