Cannot get value from sensor as Android background service

I need to build an android app which needs to get step sensor in background, so that even the app is closed it can still return the step count.

I make a java plug-in with android studio. I can get the static value from it. However the sensor value has to be non-static and sharedprefs also need to be non-static. Is that I cannot get non-static value from java class?

Here is the code:

(Java Plug-in On Android Studio Side)

 public class My_Plugin extends Activity {
   
      //Testing: Unity can get the words successfully
       public static String getMessage()
       { return "Hello Android!"; }
   
       //sensor
       public float stepCount = 0;
       public float stepDetect = 0;
       private SensorManager mgr;
       private Sensor step_counter_sensor;
       private Sensor step_detector_sensor;
   
       SharedPreferences settings;
       String pref = "pref";
       SharedPreferences.Editor editor;
   
 
      //functions call by Unity
       public float checkPrefs(){
             return settings.getFloat("Checking", 9999f);
        }
 
      public float getStepCount(){
            return stepCount;
       }
     
   @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
   
          //save a value "100" which pass to Unity
           settings = getSharedPreferences(pref,0);
           editor = settings.edit();
           editor.putFloat("Checking", 100);
           editor.apply();
   
          //Sensor management
          mgr = (SensorManager) this.getSystemService(SENSOR_SERVICE);
 
         step_counter_sensor = mgr.getDefaultSensor(Sensor.TYPE_STEP_COUNTER);
         if (step_counter_sensor == null) // sensor not supported
             Log.d("SensorTest", "sensor of interest not detected.");
         else {
             mgr.registerListener(listener, step_counter_sensor, Sensor.REPORTING_MODE_ON_CHANGE);
         }
 
         step_detector_sensor = mgr.getDefaultSensor(Sensor.TYPE_STEP_DETECTOR);
         if (step_detector_sensor == null) // sensor not supported
             Log.d("SensorTest", "sensor of interest not detected.");
         else {
             mgr.registerListener(listener, step_detector_sensor, Sensor.REPORTING_MODE_ON_CHANGE);
         }
     }
 
     private SensorEventListener listener = new SensorEventListener() {
         @Override
         public void onSensorChanged(SensorEvent event) {
 
             if (event.sensor.getType() == Sensor.TYPE_STEP_COUNTER) {
                 stepCount = event.values[0];
             }
             else if (event.sensor.getType() == Sensor.TYPE_STEP_DETECTOR) {
 
                 if (event.values[0] == 1)
                     stepDetect++;
 
             }
         }
 
         @Override
         public void onAccuracyChanged(Sensor sensor, int accuracy) {
 
         }
     };

(App on Unity side)

  using UnityEngine;
   using UnityEngine.UI;
   using System.Collections;
   
   public class callPlugin : MonoBehaviour
   {
   
       public Text txt;
       public Text txt2;
       public Text txt3;
   
       AndroidJavaClass pluginClass;
   
       void Start()
       {
           pluginClass = new AndroidJavaClass("kanwwong2.scm.myplugin.My_Plugin");
   
                   // call testing function: Success
           txt.text = pluginClass.CallStatic<string>("getMessage");
   
                  // call to get value: Fail
                  // it should return 100 if succeed, but it gives me 0
           txt2.text = pluginClass.Call<float> ("checkPrefs").ToString();
   
                 // also fail
           txt3.text = pluginClass.Call<float> ("getStepCount").ToString();
 
       }
   }
 

I cannot get the sensor value directly nor get the shared pref value. Can anyone help me please? What should I do in order to get the step count?