Bug 1053968 - Clean up mozL10n API use in SMS

https://bugzilla.mozilla.org/show_bug.cgi?id=1053968
I recommend using DOM's setAttribute over any mozL10n API when feasible.

navigator.mozL10n.setAttributes can be used to set l10nId and l10nArgs. When you don't have l10nArgs, it's better to use DOM's setAttribute because it doesn't require mozL10n at all!

When you do have l10nArgs you can also manually use setAttribute to set l10nId and l10nArgs, but then you have to manually JSON.parse the args, so using mozL10n.setAttributes covers that implementation detail (in the future we may want to change how we store l10nArgs on a node).

tl;dr:
 - use DOM's setAttribute when you have l10nId
 - use mozL10n's setAttributes when you have l10nArgs as well