No longer constantly triggering highlights on touch devices with Graphie

Summary:
Previously, if you pressed down on an interactive graph for a second
or two, the entire graph would highlight (as if you were trying to copy-paste).
Now, the underlying Graphie is unselectable.

Test Plan:
Run Perseus on a touch device, create a click-to-add-points
interactive graph, press and hold. Note that the graph does not become
selected!

Reviewers: jack

Reviewed By: jack

Subscribers: eater

Differential Revision: http://phabricator.khanacademy.org/D9532