Move localeToFixed to khan-exercise.js

@alpert merged a pull request (commit 55a546bce5a806664eb9ab349684033335348aab) that added a use of localeToFixed in utils/answer-types.js. Since localeToFixed was defined in utils/math-format.js, it wasn't always available for all exercises. Moving it to khan-exercise.js so it's always available.

Test plan: Load an exercise that doesn't use math-format (addition_1) in local mode and observe there's no javascript error from answer-types.js

Auditors: alpert, csilvers