NetworkAgent是在数据业务创建成后才创建的.

1.mDstMessenger是NetworkAgent, mSrcHandler是ConnectivityService里面的mTrackerHandler

2.在ConnctivityService里面的会对mNetworkAgentInfos和mNetworkForNetId进行赋值

private void handleRegisterNetworkAgent(NetworkAgentInfo na) {

    if (VDBG) log("Got NetworkAgent Messenger");

    mNetworkAgentInfos.put(na.messenger, na);

    synchronized (mNetworkForNetId) {

        mNetworkForNetId.put(na.network.netId, na);

    }

    na.asyncChannel.connect(mContext, mTrackerHandler, na.messenger);

    NetworkInfo networkInfo = na.networkInfo;

    na.networkInfo = null;

    updateNetworkInfo(na, networkInfo);

}