In the romantic drama “My Policeman,” wildly charismatic pop singer Harry Styles plays a 1950s police officer married to a woman but in love with a man. Sounds scandalous! But this dud of a romance is as dour as its monochromatic color palate. And it uses a boneheaded framing device that removes any sense of urgency from its telling, rendering the love triangle at its center limp and inert. At least Styles wears his period fashions well. He plays Tom Burgess, a cop in Brighton, England, who meets schoolteacher Marion (Emma Corrin) and has a polite if not exactly burning hot relationship with h…