In “The Sky Is Falling: How Vampires, Zombies, Androids, and Superheroes Made America Great for Extremism,” cultural journalist Peter Biskind dives headlong into two decades of popular culture, from superhero franchises and series like “The Walking Dead” and “Game of Thrones” to thrillers like “Homeland” and “24,” and emerges to argue that these shows are saturated with the values that are currently animating our extreme politics.