Bug 1053968 - Clean up mozL10n API use in SMS

https://bugzilla.mozilla.org/show_bug.cgi?id=1053968
Could you please replace this with?:
```
sinon.assert.calledWith(
  navigator.mozL10n.setAttributes,
  contactName, 'thread-header-text', { name: 'Pepito O\'Hare', n: 0 }
);
```