Make VerifyCredentialResponse#RESPONSE_* private

To help prevent bugs when checking a VerifyCredentialResponse, all
direct uses of the response code have been replaced with helper methods.
Therefore, the response code values no longer need to be public, nor
does the method to retrieve the response code.

Bug: 423038471
Test: atest FrameworksServicesTests:com.android.server.locksettings
Flag: EXEMPT refactor
Change-Id: If683f4c36e43f122eca08a09506bd03711dc5a0c