“Talk about understatements: When Matt Berninger sings, “I can’t fight it any more/ ’m going through an awkward phase,” on the National’s new single, “Demons,” it seems less about whatever may have happened in the three years between High Violet and the forthcoming Trouble Will Find Me than a wry remark on his band’s entire anxious 14-year existence—and how they’ve stopped trying to second-guess it”