Blur MathOutput on click, rather than mouseDown

Summary: On touch devices, MathOutputs were blurring when you clicked to drag (e.g.,
to scroll down the webview), making it impossible to keep an input focused while
scrolling down the page. The interaction is much more natural if you blur only on
click.

Test Plan:
- Run mobile app (with this build).
- Try "Groups of tens".
- Mouse-down and scroll; input should not blur.
- Click normally; input should blur.

Auditors: jack, kasra