Bug 1053968 - Clean up mozL10n API use in SMS

https://bugzilla.mozilla.org/show_bug.cgi?id=1053968
There are two reasons why we use get/setAttribute over dataset. 

First one is that the current attribute names will change when we will standardize mozL10n as a WebAPI to l10n-id and l10n-attrs.
Second is the performance. :)