Incorrect listener readiness reporting

In the process of debugging #11826 , I found that the usual stats for readiness are mis-leading. It can happen that the initial init manager finishes initializing (LDS/CDS accepted, processed, and active), yet the connections will be refused. This is because there is a delay between when init manager finishes and the workers start accepting traffic. I think this is a problem, since it may lead to pre-mature deployment of a proxy before it is ready to serve traffic.

cc @lambdai