You missed the content type and json, check below:
URL url = new URL("http://build-master-mobile.xxx.com:8080/job/Reprovision-IPA/build");
URLConnection urlConnection = url.openConnection();
HttpURLConnection httpConn = (HttpURLConnection)urlConnection;
httpConn.setRequestProperty("Accept", "application/json");
httpConn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
httpConn.setRequestProperty("Connection", "keep-alive");
httpConn.setRequestMethod("POST");
String data = "json='{\"parameter\":[{\"name\":\"IPA_URL\",\"value\":\"a\"},{\"name\":\"IPA_FILENAME\",\"value\":\"a.ipa\"}]}'";
httpConn.setDoOutput(true);
httpConn.setDoInput(true);
// send request
OutputStreamWriter wr = new OutputStreamWriter(httpConn.getOutputStream());
wr.write(data.toString());
wr.flush();
wr.close();
BufferedReader rd = new BufferedReader(new InputStreamReader(httpConn.getInputStream()));
while ((line = rd.readLine()) != null) {
result.append(line);
}
rd.close();