Consistent country code for new addresses

When adding a new address in settings, the default value of the country
select is determined using PDM::GetDefaultCountryCodeForNewAddress().
It returns either the ADM::MostCommonCountryCodeFromProfiles() or falls
back to GetCountryCodeForExperimentGroup(). The latter function
considers the variation country code, the country code of the current
time zone and the country of the app locale, in this order.
In addition, there is some caching going on, ensuring that the most
common country doesn't change when profiles are added.

On form import, the logic works different. It prefers the country
observed in the submitted form and falls back to either the variation
country code or the app locale.

This CL simplifies the logic used for settings to match the form import
logic (that is, only consider variation country code and app locale).

In particular, this CL:
- Removes ADM::MostCommonCountryCodeFromProfiles() and the cached value
  in ADM::most_common_country_code_.
- Moves PDM::GetDefaultCountryCodeForNewAddress() to the ADM and
  simplifies its implementation to only consider variation country code
  and app locale. To do so, the variation country code is passed in to
  the ADM via its constructor.
  The function now returns an AddressCountryCode (strong alias for
  std::string) instead of std::string.
- Removes the tests for GetDefaultCountryCodeForNewAddress(), which are
  concerned about the (now removed) caching behavior.
- Updates FormDataImporter::ComplementCountry() to use
  ADM::GetDefaultCountryCodeForNewAddress(), instead of duplicating the
  logic to derived the fallback country code. This makes
  ADM::GetDefaultCountryCodeForNewAddress() the single source of truth.
  This regresses the output to autofill-internals somewhat, since only
  the fallback country code is logged, not how it was derived.
- Removes GetPredictedCountryCode(), which was previously used in the
  form import logic.

Since PDM::GetDefaultCountryCodeForNewAddress() is moved to the ADM,
the few existings calls are forwarded to the ADM using the PDM's
address_data_manager() getter.

Bug: b/322170538
Change-Id: I5f8dba64ac00eb9add2c6a924b0533bb79a46251
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5439418
Reviewed-by: Dominic Battre <battre@chromium.org>
Reviewed-by: Jan Keitel <jkeitel@google.com>
Commit-Queue: Florian Leimgruber <fleimgruber@google.com>
Cr-Commit-Position: refs/heads/main@{#1286372}