Autofill sample: added icons, fixed an auth bug, added more comments.

* Added icons for autofill popups.
* Fixed RTE in the service due to autofill being enabled on the autofill login page.
* Added more comments to other unrelated functions.
* Added helper functions for ensuring only supported hints are supported.
* Added string resources.
* Changed some method/variable names to match newish class names.

Bug: 38182790
Test: manual
Change-Id: I1874c7be12ee1b50f5cc94542561d00820ba1874