Make getSharedPrefsNameByChannelId private

Author: aurimasc

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

@@ -185,7 +185,7 @@ public void registerNotificationChannel(
         }
     }
 
-    protected static String getSharedPrefsNameByChannelId(String id)
+    private static String getSharedPrefsNameByChannelId(String id)
     {
         return String.format("unity_notification_channel_%s", id);
     }