Expose content-available APS key for iOS silent push

Summary:
<details>
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_What existing problem does the pull request solve?
In iOS when sending a silent push notification you need to configure the 'content-available' APS key to the value of 1 (When this key is present, the system wakes up your app in the background and delivers the notification to its app delegate, see [apple docs](https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/PayloadKeyReference.html#//apple_ref/doc/uid/TP40008194-CH17-SW1)).

This PR exposes this property to the notification event handler so app code can handle silent push scenario specifically. Currently this property is not available.

I've updated the PushNotificationIOSExample in the RNTester.

1. Open RNTester in xcode
2. Enable the  push notifications capability
3. run on device
4. Go to PushNotificationIOS
5. click on "send fake notification"
6. verify alert message contains 'content-available' with a value of 1.
Closes https://github.com/facebook/react-native/pull/14584

Differential Revision: D5279181

Pulled By: shergin

fbshipit-source-id: d2288e147d89ba267f54265d819aa0a9969095e7

Libraries/PushNotificationIOS/PushNotificationIOS.js

@@ -24,6 +24,8 @@ const NOTIF_REGISTER_EVENT = 'remoteNotificationsRegistered';
 const NOTIF_REGISTRATION_ERROR_EVENT = 'remoteNotificationRegistrationError';
 const DEVICE_LOCAL_NOTIF_EVENT = 'localNotificationReceived';
 
+export type ContentAvailable = 1 | null | void;
+
 export type FetchResult = {
   NewData: string,
   NoData: string,
@@ -122,6 +124,7 @@ class PushNotificationIOS {
   _alert: string | Object;
   _sound: string;
   _category: string;
+  _contentAvailable: ContentAvailable;
   _badgeCount: number;
   _notificationId: string;
   _isRemote: boolean;
@@ -419,6 +422,7 @@ class PushNotificationIOS {
           this._sound = notifVal.sound;
           this._badgeCount = notifVal.badge;
           this._category = notifVal.category;
+          this._contentAvailable = notifVal['content-available'];
         } else {
           this._data[notifKey] = notifVal;
         }
@@ -484,6 +488,13 @@ class PushNotificationIOS {
     return this._alert;
   }
 
+  /**
+   * Gets the content-available number from the `aps` object
+   */
+  getContentAvailable(): ContentAvailable {
+    return this._contentAvailable;
+  }
+
   /**
    * Gets the badge count number from the `aps` object
    */

RNTester/js/PushNotificationIOSExample.js

@@ -72,11 +72,13 @@ class NotificationExample extends React.Component {
 
   _sendNotification() {
     require('RCTDeviceEventEmitter').emit('remoteNotificationReceived', {
+      remote: true,
       aps: {
         alert: 'Sample notification',
         badge: '+1',
         sound: 'default',
-        category: 'REACT_NATIVE'
+        category: 'REACT_NATIVE',
+        'content-available': 1,
       },
     });
   }
@@ -115,9 +117,15 @@ class NotificationExample extends React.Component {
   }
 
   _onRemoteNotification(notification) {
+    const result = `Message: ${notification.getMessage()};\n
+      badge: ${notification.getBadgeCount()};\n
+      sound: ${notification.getSound()};\n
+      category: ${notification.getCategory()};\n
+      content-available: ${notification.getContentAvailable()}.`;
+
     AlertIOS.alert(
       'Push Notification Received',
-      'Alert message: ' + notification.getMessage(),
+      result,
       [{
         text: 'Dismiss',
         onPress: null,