Object Mask tool -> Stroke

Object Mask only creates an alpha channel. If the Alpha Channel is clean, you can use Layer/Auto Trace to create an animated mask from it.

You can then use the Pen tool to create a new shape layer with a single click, reveal the Shape Layer path (uu) and the mask in your Object Mask layer, and simply drag the pickwhip to the shape layer mask. 

Now you can turn off the mask to keep the Object mask and then use any of the path animation tools available to shape layers to animate the stroke, color, or other properties. 

It looks something like this when completed:

Have you tried applying the stroke effect to the masked layer and adjusting the start or end parameters in the Effect Controls panel?