Add toJSONObject method to OSInAppMessage

- Add this method for the wrappers to use
- `OSInAppMessageInternal` extends `OSInAppMessage` so its `toJSONObject` method must be `public` because we can't override to reduce visibility

Author: nan-li

OneSignalSDK/onesignal/src/main/java/com/onesignal/OSInAppMessage.java

@@ -29,8 +29,13 @@
 
 import androidx.annotation.NonNull;
 
+import org.json.JSONException;
+import org.json.JSONObject;
+
 public class OSInAppMessage {
 
+    public static final String IAM_ID = "messageId";
+
     /**
      * The unique identifier for this in-app message
      */
@@ -46,4 +51,16 @@ public String getMessageId() {
         return messageId;
     }
 
+    public JSONObject toJSONObject() {
+        JSONObject mainObj = new JSONObject();
+        try {
+            mainObj.put(IAM_ID, messageId);
+        }
+        catch(JSONException e) {
+            e.printStackTrace();
+        }
+
+        return mainObj;
+    }
+
 }

OneSignalSDK/onesignal/src/main/java/com/onesignal/OSInAppMessageInternal.java

@@ -145,7 +145,8 @@ protected ArrayList<ArrayList<OSTrigger>> parseTriggerJson(JSONArray triggersJso
         return parsedTriggers;
     }
 
-    JSONObject toJSONObject() {
+    @Override
+    public JSONObject toJSONObject() {
         JSONObject json = new JSONObject();
 
         try {