Initial framework support for iOS platform views. (#23412)

This PR adds the full framework stack (layer->render object->widget, and
service) for embedding iOS views with minimal functionality.

I allowed myself to throw the entire framework stack in one PR since we're mostly
mirroring the structure we already established for embedded Android views, so this PR
is a little bigger than usual. I'm happy to break it down to the
different pieces of the stack if reviewers prefer.

Specifically this PR adds:
* A UiKitView widget for embedding a UIView in the widget tree.
* A RenderUiKitView which is the render object for showing a
  UIView.
* A PlatformViewLayer which denotes the position of a UIView in the
  layer tree.
* The iOS platform_views system channel client code in services/platform_views.dart
* Splits the fake platform views controller to an Android and iOS
  controllers.

TBD in following PRs:
* Plumb the layout direction all the way to the platform view (currently
  there is still no engine support for it).
* Integrate with gesture arenas (engine support is still missing as
  well).