Even in a decade of constant change for Bowie, the years 1975 and 1976 were transformative. With hits like “Fame†and “Young Americans,†he’d achieved the American superstardom he’d long desired -- and tired of it just as quickly, abandoning Hollywood and soul music for Berlin and dark, pioneering electronic-based music. Andrew Kent was Bowie’s photographer during this period, and his new book “Behind the Curtain†offers a strikingly intimate look at an artist during a…