Rarely has the phrase “torn from the headlines” burned with as much relevance as it does with Grounded, the mesmerizing show that opened Sunday at the Public Theater. George Brant’s one-woman play about America’s shift to drone warfare may have been written four years ago (and performed elsewhere around the country), but its divisive subject is as current as the front page of today’s New York Times. And its relevance is heightened by the fact that a war perceived…