Cannot convert Json array to Json Object in Android?

This getAppData method in Android gives me a blank screen but if I don't convert it to Json Object it gives me this;

["x.apk","xx.apk","xxx.apk"]

but I want the output to be without quotation and square brackets how can achieve this ?

PHP

$fileList = glob('xxxx/xxx/appstore_php/*');
   $files = [];
   foreach($fileList as $filename)
   {
      if ( is_file ( $filename ) )
      {
         $bname = basename ( $filename);
         $files[] = $bname;  
      }
   }

   header('Content-Type: application/json');
   $response = ["files" => $files];
   echo json_encode($response);

Output

   {"files":["x.apk","xx.apk","xxx.apk"]}

Android

    private void getAppData(){
        //Creating a string request
        StringRequest stringRequest = new StringRequest(URLAddress.SHOW_ALL_APK,
            new Response.Listener<String>() {
                @Override
                public void onResponse(String response) {
                    JSONObject j = null;
                    try {
                        JSONObject json = new JSONObject(response);
                        JSONArray jsonArr = json.getJSONArray("files");
                        for (int i = 0; i < jsonArr.length(); i++) {
                            j = jsonArr.getJSONObject(i);
                            appList.add( j.toString() );
                        }
                    } catch (JSONException e) {
                        e.printStackTrace();
                    }
                }
            },
            new Response.ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                }
            });
        //Creating a request queue
        RequestQueue requestQueue = Volley.newRequestQueue(this);

        //Adding request to the queue
        requestQueue.add(stringRequest);
    }