Bug 821604 - Don't start periodic sync until we have a calendar that needs a real sync operation r=millermedeiros

maybe `db` was needed because of `settingsStore`? if so maybe stub the `settings.getValue`?

```js
sinon.stub(settings, 'getValue')
  .withArgs('syncFrequency').returns(Promise.resolve(1))
  .withArgs('syncAlarm').returns(Promise.resolve(null));
```