Bug 1043615 - Clean up mozL10n.get uses in Settings app

https://bugzilla.mozilla.org/show_bug.cgi?id=1043615
do we have a list of potential values? It seems very risky and unpredictible to change behavior depending on if a localizer provided a translation. Also, in the future we will want to expose l10nId in case of missing string (it's more informative than empty string for users, and the UI is actionable).