Bug 1053968 - Clean up mozL10n API use in SMS

https://bugzilla.mozilla.org/show_bug.cgi?id=1053968
Not related to this patch, but just curious: I see that in l10n lib you use ```getAttribute``` to retrieve data attribute value, but in SMS app we mainly use ```dataset``` accessor. Just compared its performance (eg. http://jsperf.com/dataset-vs-getattribute-and-setattribute/3) and it looks like ```getAttribute``` approach is much(?) faster. So the question is: do you intentionally use ```getAttribute``` knowing about  any performance/other implications of ```dataset``` and we should avoid ```dataset``` too?