Replaced auto-fill by autofill to keep it consistent with API style.

To keep backward compatibility, it looks for the new service (AutofillService),
then falls back to the old one (AutoFillService) if not found.

Bug: 35956626
Bug: 36024626
Test: manual verification with a custom service
Test: RunSettingsRoboTests passes

Change-Id: I64112ac5d796e94866e632ec8ea79a3fb13e3d9b