[Autofill] Relax the requirements for credit card import

Before this CL, a necessary requirement for credit card import
was that there are no two fields of the same type with non-empty
value.

This CL weakens this requiments as follows.

Firstly, we drop the above requirement. Instead, we now check if
the values are equivalent. If not, we reject import.

Secondly, we now rank fields by priority: (1) user-edited fields,
(2) autofilled fields, (3) others. Fields of different priority
may have inconsistent values: the lower priority value is ignored.

For example, consider a form
  Full name:  <input autocomplete=name>
  First name: <input autocomplete=given-name>
  Last name:  <input autocomplete=family-name>

Scenario 1: the user typed "Donald Trump", "Donald", and "Trump",
respectively. Then we offer import because the the values are
consistent.

Scenario 2: the user typed "Donald J. Trump" and autofilled "Donald"
and "Trump". Then we offer import for "Donald J. Trump" because
typing trumps autofilling.

Bug: 1381477
Change-Id: I79db1e2879049c219385b69ff981426d512c8e8e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5018128
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Dominic Battre <battre@chromium.org>
Commit-Queue: Christoph Schwering <schwering@google.com>
Cr-Commit-Position: refs/heads/main@{#1238499}