Bug 1040271 - [L10n][SMS] Clean up mozL10n.get uses in SMS app

nit: indentation fixes:
```javascript
assert.equal(
  navigator.mozL10n.getAttributes(simDetail).id,
  'sim-detail-full
);
```

But it's still easier to just stub getAttributes using sinon:
```javascript
// in a setup function
this.sinon.stub(navigator.mozL10n.getAttributes);

// in the test
sinon.assert.calledWithMatch(
  navigator.mozL10n.getAttributes,
  {
    id: 'sim-detail-full',
    args: {
      operator: ...,
      number: ...
    }
);
```