Mathew Rosengart, a powerhouse litigator whose resume includes clerking for U.S. Supreme Court Justice David Souter and a stint as a Department of Justice trial attorney, has a stable of high-profile Hollywood clients like Steven Spielberg, Michael Mann, Sean Penn and Casey Affleck. But it’s his work with Britney Spears in successfully freeing the pop superstar from a controversial, restrictive and highly scrutinized 13-year conservatorship that catapulted the Greenberg…