Add xamarin way to use OSRemoteNotificationReceivedHandler#381

tmijieux wants to merge 2 commits intoOneSignal:mainOneSignal/OneSignal-Xamarin-SDK:mainfrom

tmijieux:use_OSRemoveNotificationReceivedHandler_in_xamarintmijieux/OneSignal-Xamarin-SDK:use_OSRemoveNotificationReceivedHandler_in_xamarinCopy head branch name to clipboard