In Mike Mills’ “20th Century Women,” Annette Bening plays Dorothea, the 55-year-old single mother of 15-year-old Jamie, played by Lucas Jade Zumann. (Jamie’s absent father can be found in Mills’ previous movie, “Beginners,” in which a formerly closeted 75-year-old comes out as gay to his son.)Jamie and Dorothea live in Santa Barbara, Calif. The year is 1979. The Carter administration is in its death throes. It is that moment between the Vietnam War and the AIDS crisis when the …