The Americanization of Emily
LEAD: Two years ago, Emily Lloyd might have been dismissed as one of those minor storm systems that rattle the showbiz barometers, stir the moviegoing public and evaporate as suddenly and inexplicably as they arrived. After all, she was only 16 and had appeared in a single low-budget film. She'd played the brazen adolescent who scandalizes a sedate British seaside town with her sexual bravado in ''Wish You Were Here,'' and she'd done nothing else whatsoever.