AP Film Writer PARK CITY, Utah (AP) — “Who’s that girl?” was a common refrain from Sundance Film Festival goers walking out of “My Old Ass” in January. Its vibrant star, Maisy Stella, was a child sensation who stepped away from the limelight for a few years to be a normal teenager. Now, at 20, she’s ready to get back to work and is already breaking through in a big way, having already booked a role in a high-profile Anne Hathaway and Ewan McGreg…