QuickStart: Fix FIDO credential id encoding

FIDO credential id is treated as Base64Url encoded (not standard Base64)
on the Android side. Since Gaia is able to gracefully deal with
Base64Url and Base64 both, prefer Base64Url encoding for FIDO credential
id.

Bug: b/289996415
Test: unit_tests --gtest_filter="*ConnectionTest*"
Test: unit_tests --gtest_filter="*SecondDeviceAuthBrokerTest*"
Change-Id: Ib59f58e2f5e3fa371d9dc3ff3e786686a39991e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5066249
Reviewed-by: Renato Silva <rrsilva@google.com>
Commit-Queue: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Michael Hansen <hansenmichael@google.com>
Cr-Commit-Position: refs/heads/main@{#1231751}