Set host of development server for setupDevtools

Summary:
Related to #15126, and this would be useful for use React DevTools on real device without modify `setupDevtools.js`.

In Android emulator, the host of `SourceCode.scriptURL` is same with `PlatformConstants.ServerHost` so we can just replace it.

* Tested on iOS device with [react-devtools](https://github.com/facebook/react-devtools/tree/master/packages/react-devtools) package.
* Tested on Android emulator, the `getDevServer` module got the correctly hostname so that don't need `adb reverse`.

[ENHANCEMENT] [setupDevtools] Set host of development server for setupDevtools
Closes #15547

Differential Revision: D6544980

Pulled By: javache

fbshipit-source-id: a286874bcef0501c5d2e0be2251d58c236a5534a

Author: jhen0409

Libraries/Core/Devtools/setupDevtools.js

@@ -28,11 +28,11 @@ let register = function () {
 if (__DEV__) {
   const AppState = require('AppState');
   const WebSocket = require('WebSocket');
-  const {PlatformConstants} = require('NativeModules');
   /* $FlowFixMe(>=0.54.0 site=react_native_oss) This comment suppresses an
    * error found when Flow v0.54 was deployed. To see the error delete this
    * comment and run Flow. */
   const reactDevTools = require('react-devtools-core');
+  const getDevServer = require('getDevServer');
 
   // Initialize dev tools only if the native module for WebSocket is available
   if (WebSocket.isAvailable) {
@@ -42,10 +42,11 @@ if (__DEV__) {
     // or the code will throw for bundles that don't have it.
     const isAppActive = () => AppState.currentState !== 'background';
 
-    // Special case: Genymotion is running on a different host.
-    const host = PlatformConstants && PlatformConstants.ServerHost ?
-      PlatformConstants.ServerHost.split(':')[0] :
-      'localhost';
+    // Get hostname from development server (packager)
+    const devServer = getDevServer();
+    const host = devServer.bundleLoadedFromServer
+      ? devServer.url.replace(/https?:\/\//, '').split(':')[0]
+      : 'localhost';
 
     reactDevTools.connectToDevTools({
       isAppActive,