Limit biometric enrollments when Unicorn account is present.

This prevents biometric enrollment from happening after consent
has been obtained and is being managed by family link.

Note that this requires a corresponding change the setup wizard app
to work in most cases.

Bug: 193577587
Test: manual (enroll then relaunch SuW and repeat)
Change-Id: If260e49f38a141931d7f3362c1faf80ee7790232