Blur in _setCurrentFocus (just in-case)

Summary:
Added a call to `blurPath` when focusing to protect against multiple focuses at once.
In general, the input elements themselves should handle focusing and blurring. However,
given that we want responsiveness for <MathOutput>s, it makes sense to put their focus
event in onMouseDown/onTouchStart. At the same time, we don't want them to blur if you're
scrolling; so, to avoid having to do weird touch tracking with jQuery, I'd prefer to put the blur
in the onClick handler. This discrepancy requires a failsafe like the one I've added here.

Test Plan:
- Run mobile app.
- Open "Groups of tens".
- Click one input. Click another. Make sure basic blurring and focusing works.
- Drag the scroll view and click another input as it snaps back into place. The previously-focused input should blur.

Reviewers: jack

Reviewed By: jack

Subscribers: kasra

Differential Revision: http://phabricator.khanacademy.org/D11837