Dependency Inject Movable into MovablePoint

Summary: Hopefully this will make it easier to test MovablePoint

Test Plan:
 * make an interactive-graph with one or two points
 * make sure
    * you can move them
    * they don't overlap each other
    * the coordinates of the answer change when you move the
      solution
    * grading works for trivial cases

Reviewers: justinjaffray

Reviewed By: justinjaffray

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D7118