title: Periodic work requests using WorkManager not working
body: <p>i am trying to write a periodic workmanager script but it just run when i open the app and it just run one time (not periodic) !</p>
  
  <p>here is my main activity :</p>
  
  <pre><code>public class MainActivity extends AppCompatActivity {
  
  @Override
  protected void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
      setContentView(R.layout.activity_work);
  
      Intent intent = new Intent();
      PendingIntent pendingIntent = PendingIntent.getActivity(MainActivity.this,0,intent,0);
      NotifyWorker.pendingIntent = pendingIntent;
      NotifyWorker.context = this;
  
      PeriodicWorkRequest periodicWorkRequest = new PeriodicWorkRequest.Builder(NotifyWorker.class, 1, TimeUnit.MINUTES).build();
      WorkManager.getInstance().enqueue(periodicWorkRequest);
  }
  </code></pre>
  
  <p>}</p>
  
  <p>and this is my dowork method :</p>
  
  <pre><code>public Result doWork() {
      Log.i("wd","wd");
  
      NotificationCompat.Builder notificationBuilder = new NotificationCompat.Builder(context,"ctx")
              .setSmallIcon(R.mipmap.ic_launcher)
              .setLargeIcon(BitmapFactory.decodeResource(context.getResources(),R.mipmap.ic_launcher))
              .setSmallIcon(R.drawable.logo)
              .setContentTitle("Title")
              .setContentText("Desc")
              .setContentIntent(pendingIntent);
  
      android.app.NotificationManager notificationManager =
              (android.app.NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
  
      notificationManager.notify(0 , notificationBuilder.build());
  
      return Result.SUCCESS;
  }
  </code></pre>
  
  <p>why its not run every 1 minute ? what i miss ?</p>