Add deep linking support to IntentAndroid

Summary:
Add a method to handle URLs registered to the app,

```js
IntentAndroid.getInitialURL(url => {
    if (url) {
        // do stuff
    }
});
```

Refer - http://developer.android.com/training/app-indexing/deep-linking.html#adding-filters

The API cannot be same as the iOS API (i.e. as a constant), as the activity is not availble at the time of module initialization. Moreover, multiple activties can share the same bridge instance, and the activity itself is not a constant. Hence the initialURL can change.
Closes https://github.com/facebook/react-native/pull/4320

Reviewed By: svcscm

Differential Revision: D2759667

Pulled By: foghina

fb-gh-sync-id: b725231ae1401fa5565d444eee5a30d303e263ae

Libraries/Components/Intent/IntentAndroid.android.js

@@ -16,6 +16,26 @@ var invariant = require('invariant');
 /**
  * `IntentAndroid` gives you a general interface to handle external links.
  *
+ * ### Basic Usage
+ *
+ * #### Handling deep links
+ *
+ * If your app was launched from an external url registered to your app you can
+ * access and handle it from any component you want with
+ *
+ * ```
+ * componentDidMount() {
+ *   var url = IntentAndroid.getInitialURL(url => {
+ *     if (url) {
+ *       console.log('Initial url is: ' + url);
+ *     }
+ *   });
+ * }
+ * ```
+ *
+ * NOTE: For instructions on how to add support for deep linking,
+ * refer [Enabling Deep Links for App Content - Add Intent Filters for Your Deep Links](http://developer.android.com/training/app-indexing/deep-linking.html#adding-filters).
+ *
  * #### Opening external links
  *
  * To start the corresponding activity for a link (web URL, email, contact etc.), call
@@ -75,6 +95,20 @@ class IntentAndroid {
     IntentAndroidModule.canOpenURL(url, callback);
   }
 
+  /**
+   * If the app launch was triggered by an app link with {@code Intent.ACTION_VIEW},
+   * it will give the link url, otherwise it will give `null`
+   *
+   * Refer http://developer.android.com/training/app-indexing/deep-linking.html#handling-intents
+   */
+  static getInitialURL(callback: Function) {
+    invariant(
+      typeof callback === 'function',
+      'A valid callback function is required'
+    );
+    IntentAndroidModule.getInitialURL(callback);
+  }
+
   static _validateURL(url: string) {
     invariant(
       typeof url === 'string',