Related to https://crbug.com/854350, we would like to improve the handling of network switching in CastMediaSinkServiceImpl.  In addition to changing the network monitor to key WiFi networks on BSSID, we'd like to close cast channels to devices that have not been discovered by mDNS after some timeout starting from the network change event.  This should prevent connections to devices that are still technically reachable but are on a different subnet/VLAN from persisting.