Restore AppCompat and Fragment nullability annotations

[Copilot]

AppCompat 1.8.0 and recent Fragment bindings lose upstream JSpecify non-null annotations, causing CS8600 warnings for valid assignments such as AlertDialog.Builder.Create() and Fragment.GetString().

• Binding contracts
  • Add scoped metadata preserving JSpecify NonNull annotations in AppCompat, AppCompat Resources, and Fragment.
  • Preserve explicitly nullable parameters and return values.
• Package updates
  • Bump AppCompat and AppCompat Resources to 1.8.0.1, and Fragment to 1.9.1.1.
  • Update public API snapshots and package metadata.
• Regression coverage
  • Add compiler coverage for non-null results and nullable inputs.

These assignments should no longer produce CS8600:

AlertDialog dialog = new AlertDialog.Builder(context).Create();
string text = fragment.GetString(resourceId);

• Fixes Regression: incorrect nullability annotations in Xamarin.AndroidX.AppCompat 1.8.0 #1558

[jonathanpeppers]

I think this is an incorrect fix, we could consider backporting this to .NET 10 instead:

• dotnet/java-interop#1483