Add "type forwarders" from Xamarin.AndroidX.SavedState to Xamarin.AndroidX.SavedState.SavedState.Android (#1183)

Context: https://learn.microsoft.com/dotnet/standard/assembly/type-forwarding
Fixes: #1182
Related: #1177

This PR adds C# type forwarders to improve API compatibility between
the `Xamarin.AndroidX.SavedState` and
`Xamarin.AndroidX.SavedState.SavedState.Android` packages.

## Changes Made

Created
`source/androidx.savedstate/savedstate/Additions/TypeForwarders.cs`
with type forwards for the following types from
`Xamarin.AndroidX.SavedState` to
`Xamarin.AndroidX.SavedState.SavedState.Android`:

- `AndroidX.SavedState.ISavedStateRegistryOwner`
- `AndroidX.SavedState.SavedStateRegistry`
- `AndroidX.SavedState.SavedStateRegistryController`
- `AndroidX.SavedState.ViewTreeSavedStateRegistryOwner`

## Technical Details

The implementation follows the established pattern used in other
AndroidX projects (such as
`androidx.lifecycle/lifecycle-viewmodel/Additions/TypeForwards.cs`).
The type forwarders use the
`System.Runtime.CompilerServices.TypeForwardedToAttribute` to redirect
type resolution from the `savedstate` assembly to the
`savedstate-android` assembly where the actual implementations reside.

The project reference from `savedstate` to `savedstate-android`
already exists in the generated project file, ensuring proper
compilation and type resolution.

Author: Copilot

source/androidx.savedstate/savedstate/Additions/TypeForwarders.cs

@@ -0,0 +1,6 @@
+using System.Runtime.CompilerServices;
+
+[assembly:TypeForwardedTo (typeof (AndroidX.SavedState.ISavedStateRegistryOwner))]
+[assembly:TypeForwardedTo (typeof (AndroidX.SavedState.SavedStateRegistry))]
+[assembly:TypeForwardedTo (typeof (AndroidX.SavedState.SavedStateRegistryController))]
+[assembly:TypeForwardedTo (typeof (AndroidX.SavedState.ViewTreeSavedStateRegistryOwner))]