Include the platformViewId of PlatformViews in the semantics tree. The accessibility bridge in the engine can use this id to steal the semantics nodes from the actual platform view and stick them into Flutter's semantics tree.

It is the iOS PlatformView counter part of https://github.com/flutter/flutter/pull/28953. It reverts the change in https://github.com/flutter/flutter/pull/28953/commits/5b5d6e89ec50f8f51766fb778d0e244f183a5000 and https://github.com/flutter/flutter/pull/28953/commits/03fd797eb8955561c2654b51035175202a25da9d.

https://github.com/flutter/flutter/issues/29302