xds: channel should move to TF if listener resource is not an API listener on the client

[easwars]

The xDS client currently uses the presence of the api_listener field to determine if it should perform client-side validation checks. If this field is absent, it performs server-side validation checks. The reason for this is because the xDS client does not know whether the requested listener resource is for the client-side or server-side.

When a server-side listener resource does not contain the inbound listener configuration (i.e, address, port and filter chains), the xds-enabled grpc server moves to non-serving mode (and rejects new connection attempts in this mode).

A similar check needs to be added to the client side, i.e., if a client-side listener resource does not contain the api_listener field, the channel should be put in TRANSIENT_FAILURE and RPCs must fail.