Paso 4: Definición pushidentifier

import androidx.annotation.NonNull;
import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;

import com.adobe.marketing.mobile.MobileCore;
import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.iid.FirebaseInstanceId;
import com.google.firebase.iid.InstanceIdResult;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

registerToken();
}

void registerToken() {
FirebaseInstanceId.getInstance().getInstanceId()
    .addOnCompleteListener(new OnCompleteListener<InstanceIdResult>() {
        @Override
        public void onComplete(@NonNull Task<InstanceIdResult> task) {
            if (!task.isSuccessful()) {
                Log.w("Message App", "getInstanceId failed", task.getException());
                return;
            }

// Get new Instance ID token
String token = task.getResult().getToken();

Log.d("Got token", token);

MobileCore.setPushIdentifier(token);
}
});
}

@Override
public void onResume() {
super.onResume();
MobileCore.setApplication(getApplication());
MobileCore.lifecycleStart(null);
}

@Override
public void onPause() {
super.onPause();
MobileCore.lifecyclePause();
}
}