Kristen Stewart is happiest when she is driving. The 'Snow White and the Huntsman' actress - who is dating her 'Twilight' co-star Robert Pattinson - is at her most contended when she is away from film sets and listening to music in her car. She said: "When am I happiest? In my car with the right friend -- I like to drive and I love being in Los Angeles, it always feels like, 'Ahhh.' "So I woul…