Give static manager package visibility

Author: aurimasc

com.unity.mobile.notifications/Runtime/Android/Plugins/com/unity/androidnotifications/UnityNotificationManager.java

@@ -40,7 +40,7 @@
 import com.unity3d.player.UnityPlayer;
 
 public class UnityNotificationManager extends BroadcastReceiver {
-    protected static UnityNotificationManager mUnityNotificationManager;
+    static UnityNotificationManager mUnityNotificationManager;
 
     private Context mContext = null;
     private Activity mActivity = null;