2/n: Remove unnecessary RequestThrottledException for verify paths

verifyCredential and verifyTiedProfileChallenge return a
VerifyCredentialResponse object instead of a byte[] now. Thus,
any/all information regarding the authentication attempt is
received by callers. We no longer need to use exceptions as a
method of returning an alternate non-byte[] result.

The RequestThrottledException cannot be completely removed yet
because checkCredential returns a boolean and has no way of
returning a timeout yet. See b/161956762

Bug: 161765592

Test: Accept/Reject/Lockout the following
      1) Owner profile
      2) Managed profile with separate challenge
      3) Managed profile with unified challenge
Change-Id: Ia61c71bdde42dec34d8f2eac86d7ea964b1485c2