【Unity】バックグラウンド時にandroidプラグインからUnityの関数を呼び出す【モバイル】

いつもお世話になっております。
毎度恐縮ですが質問させていただきます。

###前提・実現したいこと
タイトルの通りバックグラウンド時にプラグインからUnityの関数を呼び出したいです。
バックグラウンドというのはアプリを立ち上げてはいるがフォーカスされていない状態（ホーム画面にいる等）

###該当のソースコード

Java
1// Unity連携部分
2public class NativeDialog {
3    private static AudioAttributes audioAttributes;
4    private static SoundPool soundPool;
5    private static int soundOne;
6
7    static public void init(Context i_context, Class i_class) {
8         audioAttributes = new AudioAttributes.Builder()
9                .setUsage(AudioAttributes.USAGE_GAME)
10                .setContentType(AudioAttributes.CONTENT_TYPE_SPEECH)
11                .build();
12
13        soundPool = new SoundPool.Builder()
14                .setAudioAttributes(audioAttributes)
15                .setMaxStreams(1)
16                .build();
17
18        soundOne = soundPool.load(i_context, R.raw.one, 1);
19    }
20
21    static  public void startService() {
22        Log.i("Unity", "START Service");
23        Activity activity = UnityPlayer.currentActivity;
24        Context context = activity.getApplicationContext();
25        activity.startService(new Intent(context, testService.class));
26    }
27
28    static public void stopService() {
29        Log.i("Unity", "STOP Servise");
30        testService.shouldContinue = false;
31    }
32
33    static public void callUnity() {
34        Log.i("Unity", "callUnity");
35        // 音を鳴らしてみる
36        soundPool.play(soundOne, 1.0f, 1.0f, 0, 0, 1);
37        UnityPlayer.UnitySendMessage("Main Camera", "sendNotification", "");
38    }
39
40    static public void sendNotification(){
41        Log.i("Unity", "START sendNotification");
42
43        // Intent の作成
44        Activity activity = UnityPlayer.currentActivity;
45        Intent intent = new Intent(activity.getApplicationContext(), _class);
46        PendingIntent pendingIntent = PendingIntent.getActivity(context, 0, intent, PendingIntent.FLAG_CANCEL_CURRENT);
47
48        Notification n = new Notification.Builder(context)
49                .setContentTitle("title")
50                .setContentText("msg")
51                .setSmallIcon(R.drawable.test)
52                .setContentIntent(pendingIntent)
53                .setAutoCancel(true)
54                .build();
55
56        NotificationManager mNotifyMgr =
57                (NotificationManager) activity.getSystemService(NOTIFICATION_SERVICE);
58
59        int mNotificationId = 001;
60        // Builds the notification and issues it.
61        mNotifyMgr.notify(mNotificationId, n);
62
63        Log.i("Unity", "END sendNotification");
64    }
65}

// Service
public class testService extends IntentService {
    public static volatile boolean shouldContinue = true;

    public testService (String name) {
        super(name);
    }

    public testService () {
        super("testService");
    }

    @Override
    protected void onHandleIntent(Intent intent) {

        shouldContinue = true;

        Log.d("NotifyIntentService","onHandleIntent Start");

        long endTime = System.currentTimeMillis() + 6*1000;
        while (System.currentTimeMillis() < endTime) {
            synchronized (this) {
                try {
                    wait(endTime - System.currentTimeMillis());
                    if (!shouldContinue) {
                        stopSelf();
                        return;
                    }

                    NativeDialog.callUnity();
                } catch (Exception e) {
                }
            }
        }
    }
}

C#
1// Unity部分
2public class StartScript : MonoBehaviour
3{
4    static AndroidJavaClass _plugin = null;
5
6    // Use this for initialization
7    void Start ()
8    {
9        Init();
10#if UNITY_ANDROID && !UNITY_EDITOR
11        _plugin = new AndroidJavaClass("com.example.dialog_plugin.NativeDialog");
12#endif
13    }
14
15    void Init()
16    {
17#if UNITY_ANDROID && !UNITY_EDITOR
18        AndroidJavaClass nativeDialog = new AndroidJavaClass("com.example.dialog_plugin.NativeDialog");
19
20        AndroidJavaClass unityPlayer = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
21        AndroidJavaObject context = unityPlayer.GetStatic<AndroidJavaObject>("currentActivity");
22
23        context.Call("runOnUiThread", new AndroidJavaRunnable(() => {
24            nativeDialog.CallStatic(
25                "init",
26                context,
27                unityPlayer
28            );
29        }));
30#endif
31    }
32
33    void sendNotification()
34    {
35        Debug.Log("unity sendNotification");
36#if UNITY_ANDROID && !UNITY_EDITOR
37			_plugin.CallStatic ("sendNotification");
38#endif
39    }
40
41    void OnApplicationPause(bool pauseStatus)
42    {
43        if (pauseStatus)
44        {
45#if UNITY_ANDROID && !UNITY_EDITOR
46			_plugin.CallStatic ("startService");
47#endif
48        }
49        else
50        {
51#if UNITY_ANDROID && !UNITY_EDITOR
52			_plugin.CallStatic("stopService");
53#endif
54        }
55    }
56}