Bug 1062798 - Replace custom JSON loaders with LazyLoader.getJSON in Settings App

Replaces custom JSON loaders with the LazyLoader.getJSON helper.
Replaced code in the app permissions, display, help, search and sound panels.
Also replaced within the APN utils module.
I would suggest to leave this as an empty function so that the test script can control the response in a simple stub without worrying about the state (`_response`). Keeping the mock object simple also makes it easier to be maintained.

ex:
```
this.sinon.stub(MockLazyLoader, 'getJSON', function() {
  return Promise.resolve(mockResponse); // mockResponse could be a global variable.
});
```