Make ASG::ShouldShowVirtualCardOption private

This function is only used internally and for testing. Therefore this CL
makes it private and adds it to the TestApi.

Later on, after some more changes to the API, we might wanna remove it
from the TestApi and test it using the public (test) API.

Bug: 326950201
Change-Id: I4b1aa55b168f39bbe0f2fc761d5796d052493d8b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5326900
Reviewed-by: Florian Leimgruber <fleimgruber@google.com>
Commit-Queue: Jihad Hanna <jihadghanna@google.com>
Cr-Commit-Position: refs/heads/main@{#1266343}