Bug 1096898 - [Usage] Make Usage app work in Mulet

If you were completely right, building a DEBUG version of Gaia will leave Cost Control unusable. Which is not the case as if you make `GAIA_OPTIMIZE=0 make reset-gaia` it continues working. This is because embedding is **always performed**. Only by commenting [line 83](https://github.com/mozilla-b2g/gaia/blob/70216cf8e35a09a6b3303119f1046b938448814c/build/webapp-optimize.js#L83) you can make Cost Control fail. This is this way because it is not part of an optimization process but a mock for the `<template>` tag.

Anyway, I think using the LazyLoader is the correct way to do it but I try to remove the sharing code between them but respecting CSS and JavaScript activation. Maybe you should file a bug and try to do this in a separated PR. Though, I don't think this will cause a great performance impact so you can leave it.

Thank you for the clarifications.