Bug 1053968 - Clean up mozL10n API use in SMS

https://bugzilla.mozilla.org/show_bug.cgi?id=1053968
Is it recommended to use ```navigator.mozL10n.setAttributes(element, l10n-id)``` instead of plain ```element.setAttribute('data-l10n-id', l10n-id)``` now? If so, can you please update the code above too (line 97 in this file, https://github.com/mozilla-b2g/gaia/pull/22939/files#diff-5fea0edf7e18d12e79d0d95ca3b2d35cL97)