Add support for global alert() on Android

Summary:
Use the new cross-platform [`Alert`](https://github.com/facebook/react-native/blob/master/Libraries/Utilities/Alert.js) API to implement global `alert()`.

public

Reviewed By: vjeux

Differential Revision: D2773589

fb-gh-sync-id: 4297c5dce6a55391af89d2675f2b9ac4dc715bb6

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -117,15 +117,11 @@ function setUpTimers() {
 }
 
 function setUpAlert() {
-  var RCTAlertManager = require('NativeModules').AlertManager;
   if (!GLOBAL.alert) {
     GLOBAL.alert = function(text) {
-      var alertOpts = {
-        title: 'Alert',
-        message: '' + text,
-        buttons: [{'cancel': 'OK'}],
-      };
-      RCTAlertManager.alertWithArgs(alertOpts, function () {});
+      // Require Alert on demand. Requiring it too early can lead to issues
+      // with things like Platform not being fully initialized.
+      require('Alert').alert('Alert', '' + text);
     };
   }
 }