Native to abstract extensions

I propose to make the methods used to convert the native variants of OSNotification, OSNotificationOpenedResult, etc. to their abstract counterparts available as extension methods.

I needed this functionality when implementing an NotificationExtenderService for Android in a Xamarin.Forms project. I think the proposed changed would allow for better discoverability of this functionality.

This change is 

@ArgusMagnus I'll take a look when I am available, in the mean time can you fix the merge conflict noted below?

Yes, I'll do it once im free

@ArgusMagnus , BUMP