I freely admit the main reason I went to “The Flash” was with eager anticipation for the return of Michael Keaton as Batman. So be it, but as “The Flash” and its time- and universe-bending plot undertook its twists and turns, I found so much more to enjoy than Keaton being back in the bat-saddle as the Caped Crusader. For his performance, I (and, it must be said, the entire preview audience) was enraptured and cheering—but this is a Flash movie