Update PreviewLayer in LayoutSubviews not Draw

[PureWeen]

Description of Change

Draw is only called once and is not called if the layout of the view changes.

We had an issue logged here
xamarin/Xamarin.Forms#9623

But you can manifest the same issue by changing the orientation on the device.

• Launch camera sample in portrait
• rotate to landscape

You will notice that without this change the preview window doesn't resize

Pull Request Checklist

• Rebased on top of master at time of the pull request.
• Changes adhere to coding standard in the sample.
• Consolidated NuGet packages across all projects in the sample (when changing existing package versions).
• Tested changes on the appropriate platforms (all platforms if changing the library code).

[davidbritch]

Thanks @PureWeen