How can I know if I'm calling a plugin correctly?

I'm trying to code a plugin to toggle fullscreen in an Android device, but when I call the method, it does nothing.

Here is my Java and C# code.

JAVA:

 import android.os.Build;
 import android.view.View;
 
 import com.unity3d.player.UnityPlayerActivity;
 
 public class FullScreen extends UnityPlayerActivity {        
     
     public int EnableFullScreen() {
      
         if (Build.VERSION.SDK_INT >= 19) { //KITKAT
      
             int flagImmersiveSticky = 4096; //View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY
      
             int flags = flagImmersiveSticky
                 | View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                 | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                 | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                 | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION // hide nav bar
                 | View.SYSTEM_UI_FLAG_FULLSCREEN; // hide status bar
      
             this.findViewById(android.R.id.content).setSystemUiVisibility(flags);
         }
         return 1;
     }
     
     public int DisableFullScreen() {
          
         if (Build.VERSION.SDK_INT >= 19) { //KITKAT
      
             int flags = View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                     | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                     | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN;
      
             this.findViewById(android.R.id.content).setSystemUiVisibility(flags);
         }
         return 2;        
     }        
 }

C#

 using UnityEngine;
 using System.Collections;
 
 public class Full : MonoBehaviour {
     bool b = false;
     AndroidJavaObject jo;
     public TextMesh tm;
 
     void Start () {
         jo = new AndroidJavaObject("com.demilp.fullscreen.FullScreen");
     }
 
     void Update () {
 
         if (Input.GetMouseButtonDown(0)){
             if(!b){
                 tm.text = jo.Call<int>("EnableFullScreen").ToString();
 
             }else{
                 tm.text = jo.Call<int>("DisableFullScreen").ToString();
             }
             b = !b;
         }
     }
 }

The TextMesh should change, but stays with the default text ("Hello World"); Should I edit the AndroidManifest?