passkeys: fail conditional create requests with setting disabled

CL:6300089 added a settings toggle for disabling automatic passkey
upgrades in GPM settings. But the underlying pref isn't currently wired
into the PasskeyUpgradeRequestController, so it has no effect.

Fail the request with a NotAllowedError if the feature is disabled.

Include-Ci-Only-Tests: true

Fixed: 400800958
Change-Id: I25f3af63b7daea7bda6955dafd96a2966127ab25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6324953
Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Auto-Submit: Martin Kreichgauer <martinkr@google.com>
Commit-Queue: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1428314}