There’s a moment in the first Frozen when Elsa (Idina Menzel), in the middle of the big “Let It Go” musical number, creates a sentient snowman in the image of a childhood plaything and simply … walks away. A few scenes later the snowman meets up with Anna (Kristen Bell) and Kristoff (Jonathan Groff) but what the snowman was doing in the time in between when he was brought to life and when he joined our heroes on their adventure has remained a mystery. Until now. “Once Upon a Snowman,” a brand-new animated short from Walt Disney Animation Studios, …