Disabled CheckBox on iOS always Tints Grey

If you run the following code the disabled checkbox will be grey.
```C#
MainPage = new ContentPage
{
Content = new StackLayout()
{
Children =
{
new CheckBox() { Color = Colors.Purple },
new CheckBox() { Color = Colors.Purple, IsEnabled = false },
}
}
};
```

Technically we should just replace the entire `Checkbox` implementation with `RadioButton` so might just be best to do that instead of trying to figure this out