The next time we see Keaton in his tighty-whities in “Birdman,” he’s dashing frantically through Times Square at night, having accidentally locked himself out of that same theatre in the middle of a performance of a Raymond Carver production that he stars in, wrote and directed. He’s swimming upstream through a river of gawking tourists, autograph seekers, food carts and street performers. But despite the chaos that surrounds him, he seems purposeful, driven and–for the first time–oddly content.