Bug 1065279 - [settings] refactor Hotspot panel with AMD pattern

well, let's make black-box testing. What I have supposed that I need to do is this:
```
var fakeCb = this.sinon.spy();
hotspotSettings.observe('hotspotSSID', fakeCb);

hotspotSettings.setHotspotSSID('testSSID');
assert.isTrue(fakeCb.calledWith('testSSID'));
```
right?

However, the `SettingsListener` module isn't loading correctly. I don't like to use the mock for this module because it does not trigger automatically the callbacks, and triggering them manually with the required value seems to be a bad practice.