Add fallback to the legacy API when auth result is unknown

If the UserConsentVerifier API returned unknown authentication result,
fallback to the legacy API and record a metric.

Bug: 40286735
Change-Id: Ib416ea5d6338f2852a6e8a2e2f494673560af264
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5614350


Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Karol Sygiet <sygiet@google.com>
Cr-Commit-Position: refs/heads/main@{#1312839}