overrideAuthority does not impact authority for JWT in jwt_token_creds

[ejona86]

65e4d9f#diff-850c920bd69ac031d7e98cab9459dec8 caused grpc/grpc#9497

[ejona86]

Root problem:

1. overrideAuthority is implemented as a TransportFactory wrapper (AuthorityOverridingTransportFactory)
2. CallCredentials handling is implemented as a TransportFactory wrapper (CallCredentialsApplyingTransportFactory)
3. 2 wraps 1, so it doesn't see the overridden authority

[ejona86]

I believe this is broken independent of whether using Channel.authority() or the CallCredentials. The test was using CallCredentials, but that indirectly uses Channel.authority().