Summary:
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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
Guy Blank
Eric Vicenti