[Autofill] Make AutofillType constructor explicit

This CL essentially reverts crrev.com/c/6263387 for the following
reasons:

- The motivation of crrev.com/c/6263387 doesn't apply anymore:
  - AttributeInstance does not inherit from FormGroup.
  - AutofillType is still around and probably will stay.

- Implicit conversions are mostly banned by the style guide. The
  exception for types that represent the same underlying value will
  be less applicable when an AutofillType represents a union type.

- Before crrev.com/c/6263387, there were many GetInfo() overloads
  in the various subclasses, even with differing definitions.
  By contrast, this CL only adds a single overload to the base
  class (FormGroup).

Bug: 432645177
Change-Id: If2a6a435a500737bd44245dee4758e680a8cf02a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6785449
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Auto-Submit: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1491958}