How can I manage session in Android login/logout?

i successfully developed one login form. But now I am not able to manage session...also i have read more session concepts.but i can't do it.please explain me in detail. This is my code. Please help how I can use Session part here.

This is my coding part:

package com.androidlogin.ws;

import org.ksoap2.SoapEnvelope;
import org.ksoap2.serialization.PropertyInfo;
import org.ksoap2.serialization.SoapObject;
import org.ksoap2.serialization.SoapPrimitive;
import org.ksoap2.serialization.SoapSerializationEnvelope;
import org.ksoap2.transport.HttpTransportSE;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

 public class AndroidLoginExampleActivity extends Activity {
 private final String NAMESPACE = "http://ws.userlogin.com";
 private final String URL = "http://111.223.128.10:8085/AndroidLogin/services/Login?wsdl";
private final String SOAP_ACTION = "http://ws.userlogin.com/authentication";
private final String METHOD_NAME = "authentication";
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    Button login = (Button) findViewById(R.id.btn_login);
    login.setOnClickListener(new View.OnClickListener() {

  public void onClick(View arg0) {
   loginAction();

   }
   });
   }

 private void loginAction(){
 SoapObject request = new SoapObject(NAMESPACE, METHOD_NAME);

    EditText userName = (EditText) findViewById(R.id.tf_userName);
    String user_Name = userName.getText().toString();
    EditText userPassword = (EditText) findViewById(R.id.tf_password);
    String user_Password = userPassword.getText().toString();

  //Pass value for userName variable of the web service
    PropertyInfo unameProp =new PropertyInfo();
    unameProp.setName("userName");//Define the variable name in the web