Minor fixes on Autofill Framework:

- Removed default implementation of onProvideAutofillVirtualStructure() that
  was using accessibility API (it's useless because without the View calling
  AutofillManager.notifyViewEntered(), it would never be triggered).
- Fixed obsolete TODOs.
- Removed obsolete service class name constant.
- Removed unused debug constant.

Test: CtsAutoFillServiceTestCases (on oc-dev) pass
Test: manual verification (on oc-dev)
Bug: 37078783
Bug: 33197203
Bug: 33802548
Bug: 35956626

Change-Id: I834d34b8af8bf0d781dc7e0ffcd6e600bfa2d183