Improved documentation for AutofillService package:

- Moved (and expanded) overall documentation from FillResponse to
  AutofillService.
- Improved SaveInfo documentation.
- Improved FillRequest documentation.
- Improved Dataset documentation.

Bug: 37567048
Test: ran 'm -j doc-comment-check-docs' and checked resulting HTML

Change-Id: I157893deac06a5ed5e1cb7fd082da485f227b9ee