Bug 973443 - [settings] refactor Display panel with AMD pattern

https://bugzilla.mozilla.org/show_bug.cgi?id=973443
I would suggest to set `onsuccess` with a wrapper function to avoid passing objects via assigning properties. For example:
```
mozActivity.onsuccess = function() {
   this._handleActivityResult(mozActivity.result.blob, secret);
}.bind(this);
```