Fix MovableLine modify semantics

Summary:
Fixes a couple subtle semantic issues with MovableLine I introduced
when changing Interactive2's update/modify semantics:

1. I wasn't passing a coord to movable.grab(), which should be an error
2. I was using local variables created in `update` to store mouse state,
   but these were getting recreated whenever a new `update` or `modify`
   happened, which is every move while in a <Graphie> component

Test Plan: Cherry-picked this on top of D10783 and saw that it worked now

Reviewers: charlie, eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D10784