Backup shared preferences file to cloud

Question

I'm trying to backup my shared preferences file, made a BackupAgentHelper -

public class MyPrefsBackupAgent extends BackupAgentHelper {
// The name of the SharedPreferences file
static final String PREFS = "favorites";

// A key to uniquely identify the set of backup data
static final String PREFS_BACKUP_KEY = "prefs";

// Allocate a helper and add it to the backup agent
@Override
public void onCreate() {
    SharedPreferencesBackupHelper helper = new SharedPreferencesBackupHelper(this, PREFS);
    addHelper(PREFS_BACKUP_KEY, helper);
}

on MainActivity -

bm = new BackupManager(getApplicationContext());

    public static void requestBackup() {
    bm.dataChanged();
}

and when saving the shared preferences calling the requestBackup method -

        editor.commit();
    MainActivity.requestBackup();

manifest as below-

    <application
    android:allowBackup="true"
    android:backupAgent="MyPrefsBackupAgent">

        <meta-data android:name="com.google.android.backup.api_key"
        android:value="AEdPqrEAAAAI1Ap52upiK6qBdhShe4Diyzti2CLyayVouApfKQ" />

it is not working, what am I doing wrong?