You could create your shape in Illustrator. Combine it with the pathfinder. Then copy it.
In After effects create a Solid and paste the shape to the solid (create mask).
Now you can put helplines to border of the shape.
Go to Mask Path and create your final keyframe.
Create a start keyframe (on Mask Path). Use the convert vertex tool for the anchors.
Drag the anchors with the selection tool to the corner of the helpline.
Hope that helps?