`<FocusZone>` fix.

If the input started out focused then the `<MathInput>` would have
`focused=true` while the `<FocusZone>` would have `focused=false`, so
they would be out of sync and the buttons wouldn't close until you click
the page, then input, then page.

Test plan:
* open http://dev:8080/math/cc-sixth-grade-math/cc-6th-expressions-and-variables/cc-6th-writing-expressions/e/writing-numerical-expressions-with-exponents-word-problems
* click the page, see buttons close

Auditors: jack