doInBackground()

 @Override
 protected String doInBackground(Context...contexts) {
  Context context = contexts[0];
  System.out.println("Got context: " + context.toString());

  OkHttpClient client = new OkHttpClient();

  System.out.println("Created new OkHttpClient");

  try {
   InputStream inputStream = context.getResources().openRawResource(R.raw.keys);
   String jsonString = new Scanner(inputStream).useDelimiter("\\A").next();
   JSONObject object = (JSONObject) new JSONTokener(jsonString).nextValue();
   String CLIENT_ID = object.optString("client_id");
   String REDIRECT_URI = object.optString("redirect_uri");
   String CLIENT_SECRET = object.optString("client_secret");
   String AUTHORIZATION_CODE = object.optString("authorization_code");
   String GRANT_TYPE = object.optString("grant_type");
   String TARGET_URL = object.optString("token_target_url");
   System.out.println("Got everything from keys.json");

   RequestBody bodyBuilder = new FormBody.Builder()
    .add("client_secret", CLIENT_SECRET)
    .add("client_id", CLIENT_ID)
    .add("code", AUTHORIZATION_CODE)
    .add("grant_type", GRANT_TYPE)
    .add("redirect_uri", REDIRECT_URI)
    .build();
   System.out.println("Built body: " + bodyBuilder.toString());

   String mediaTypeString = "application/x-www-form-urlencoded";
   MediaType mediaType = MediaType.parse(mediaTypeString);
   RequestBody body = RequestBody.create(mediaType, bodyBuilder.toString());

   Request request = new Request.Builder()
    .url(TARGET_URL)
    .post(body)
    .addHeader("content-type", mediaTypeString)
    .addHeader("cache-control", "no-cache")
    .build();

   try {
    System.out.println("Starting request.");
    Response response = client.newCall(request).execute();
    String targetUrl = request.url().toString() + bodyToString(request);
    System.out.println("request: " + targetUrl);
    String responseBodyString = response.body().string();
    System.out.println("response: " + responseBodyString);
    return responseBodyString;
   } catch (IOException ex) {
    System.out.println("IOException - take a look! " + ex);
   }

   return "Error in getting access token.";
  } catch (JSONException e) {
   e.printStackTrace();
  }

  return "Error in getting keys.json";

 }