Commit ba37aa44 workaround around a signature clash incorrectly and
turned the selector to static ones (and incorrectly set the handle)
Also fix a typo in the [Advice] attribute of the old API
https://bugzilla.xamarin.com/show_bug.cgi?id=48382
The current API use `string`, not `NSString` is added as a step forward.
The `NSString` is more correct but it does not ease discoverability (e.g.
code completion) so an enum-based overload is added (as the preferred API).
https://bugzilla.xamarin.com/show_bug.cgi?id=32535
https://bugzilla.xamarin.com/show_bug.cgi?id=44322
AVAssetDownloadUrlSession.CreateSession according to headers it
should return an AVAssetDownloadUrlSession but it is returning
an apple internal type NSURLBackgroundSession so with our
current bindings it throws an InvalidCastException, adding
ForcedTypeAttribute will create the managed type wihout
the actual typecheck. Added test verifing that the API
no longer throws.