Fix second blinking cursor on mathquill editor on iOS

Summary: Fixes the second blinking cursor seen on iOS (https://github.com/mathquill/mathquill/issues/310).

Test Plan: - Test mathquill editor on iOS and observe that there is only one cursor

Reviewers: aria, alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D12105