Get server host in setupDevtools for Android

Summary:
The `setupDevtools` for Android looks coming on [v0.30](22fbb6d46d), currently we need to run `adb reverse tcp:8097 tcp:8097`, I think get host IP (`10.0.2.2`, Genymotion: `10.0.3.2`) for Android will be better. (it can be found in `AndroidConstants` native module)
Closes https://github.com/facebook/react-native/pull/8811

Differential Revision: D3586177

Pulled By: javache

fbshipit-source-id: 3bfe04391b0fea608e4d3deab03dd376fab8727c

Libraries/Devtools/setupDevtools.js

@@ -11,10 +11,17 @@
  */
 'use strict';
 
+var Platform = require('Platform');
+var NativeModules = require('NativeModules');
+
 function setupDevtools() {
   var messageListeners = [];
   var closeListeners = [];
-  var ws = new window.WebSocket('ws://localhost:8097/devtools');
+  var hostname = 'localhost';
+  if (Platform.OS === 'android' && NativeModules.AndroidConstants) {
+    hostname = NativeModules.AndroidConstants.ServerHost.split(':')[0];
+  }
+  var ws = new window.WebSocket('ws://' + hostname + ':8097/devtools');
   // this is accessed by the eval'd backend code
   var FOR_BACKEND = { // eslint-disable-line no-unused-vars
     resolveRNStyle: require('flattenStyle'),