[AF] Add logging for why the credit card Autofill toggle is disabled

Adds UMA Autofill.CreditCard.DisabledReason.[PageLoad|Startup].

Note that due to legacy ".CreditCard.IsEnabled." metrics and the pref
name being kCreditCardEnabled, the UMA uses "CreditCard", even though
in the code base we've started calling it PaymentMethods instead (in
order to encompass IBAN and other payment-based flows).

This CL is almost entirely thanks to https://crrev.com/c/5575378 and
shares a very high amount of similarity with it.  Thanks, Wolfgang!

Bug: 306775750
Change-Id: Id8f98969e9b1b146002e8d7540a50a4d67056d62
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5725635
Reviewed-by: Vipul Koul <koulvipul@google.com>
Commit-Queue: Olivia Saul <jsaul@google.com>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Matthias Körber <koerber@google.com>
Cr-Commit-Position: refs/heads/main@{#1334406}