MathOutput now correctly handles focus and blur events

Summary:
The MathOutput component (used by InputNumber and Expression to render static TeX) now
correctly handles focus and blur events in the mobile app, highlighting on start, unhighlighting
on blur, etc.

Test Plan:
- Run the mobile app.
- Open "Groups of Tens". Verify that:
    - The first input-number is highlighted initially and the math pad is open.
    - Clicking on the next input-number highlights it, while the pad remains open.
    - Clicking away unhighlights the highlighted input.
- You should also check in Perseus that the onFocusChange events are firing correctly.

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D11282