Update AffiliationService::PrefetchChangePasswordURL

This CL updates AffiliationService::PrefetchChangePasswordURL to allow
prefetching a single change password URL at a time. No callers were
using more than one anyway so this change is pure mechanical. There
are no behavioral change.

This change simplifies implementation of a follow up CL, where change
password URL can be obtained from a main domain.

Bug: 391593406
Change-Id: I7f228c5f2dbc35b2e2f562c6c13c2cbe5038bdc6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6236980
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Viktor Semeniuk <vsemeniuk@google.com>
Cr-Commit-Position: refs/heads/main@{#1417201}
NOKEYCHECK=True
GitOrigin-RevId: 415d492c726999e3d62ef622ecc684719450e794