[controls] Brush.Foo should return immutable instances (dotnet#3824)

When profiling a `dotnet new maui` app, with this package:

https://github.com/jonathanpeppers/Mono.Profiler.Android

The `alloc` report shows:

    Allocation summary
    Bytes      Count  Average Type name
    39984        147 2 72     Microsoft.Maui.Controls.SolidColorBrush

Stack trace:

    38352 bytes from:
    (wrapper runtime-invoke) object:runtime_invoke_void (object,intptr,intptr,intptr)
    Microsoft.Maui.Controls.VisualElement:.cctor ()
    (wrapper runtime-invoke) object:runtime_invoke_void (object,intptr,intptr,intptr)
    Microsoft.Maui.Controls.Brush:.cctor ()

Reviewing the `Brush` class, there are indeed 147 `SolidColorBrush`
created on startup that are stored in fields.

But what is weird about this, is that `SolidColorBrush` is mutable!

    public Color Color
    {
        get => (Color)GetValue(ColorProperty);
        set => SetValue(ColorProperty, value);
    }

So I could literally write code like:

    Brush.Blue.Color = Colors.Red;

Blue is red! (insert evil laughter?)

I think the appropriate fix here is that all of these `static
readonly` fields should just be properties that return a new
`ImmutableBrush`. We can cache the values in fields on demand. Then
someone can't do something evil like change `Blue` to `Red`?

I reviewed WPF source code to check what they do, and they took a
similar approach:

https://github.com/dotnet/wpf/blob/5e8187344b2b561ef08b9ca2735cd89cbdd3c11e/src/Microsoft.DotNet.Wpf/src/PresentationCore/System/Windows/Media/brushes.cs#L33-L1586

We should make this API change now before MAUI is stable, and we have
the side benefit to save 39984 bytes of memory on startup?

I added tests for these scenarios, and discovered 3 typos for `Brush`
colors that listed the wrong color.