make MouseDevice mockable

The class https://github.com/dotnet/wpf/blob/main/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Input/MouseDevice.cs does not allow right now to derive from it. This prevent any Mocking library to work.

As far as I can see is it enough to change the access modifier from
`internal abstract MouseButtonState GetButtonStateFromSystem(MouseButton mouseButton);`

Code to test with Moq:
```
var t = typeof(InputManager).GetConstructor(BindingFlags.Instance | BindingFlags.NonPublic, null, new Type[0], null);
var mouseDevice = new Moq.Mock((InputManager)t.Invoke(null)).Object;
```