Deactivate default APN only on network-agent disconnection

For default apn if ConnectivityService has disabled this network,
stop trying to bring it up, but do not tear it down,
ConnectivityService will do that directly by talking with the DataConnection.

Change-Id: I2573616e7c1ebd5c1285aa1cf5d4e58de0341cc8
CRs-Fixed: 782442