Sia has not let accusations that she was “ableist” in the making of her upcoming movie, “Music,” go by without comment, and the pop star-turned-filmmaker spent the early hours of Friday tangling with disability activists who took issue with her on several points. Among the grievances she addressed, sometimes with palpable anger: her use of the term “special abilities” instead of “disabled” for people with autism; casting a non-autistic actor in the lead role of her directorial debut; and whether a person who is not autistic is exercising undue privilege in making a film centered on the subject.