Create a shared interface to access QuicTransport*Session::IsSessionReady(). Also slightly alter the logic to make sure that the session does not stop being ready when the connection is closed. gfe-relnote: n/a (not used in production) PiperOrigin-RevId: 274230704 Change-Id: Idfe2d0135e5f35b1974619066743cc9edf8c44a9