Support system accent color retrieval with UIColorType.Accent

[MartinZikmund]

What would you like to be added:

UISettings class allows retrieval of UIColorType.Accent and its darker and lighter versions. This can be supported on Android where system accent color can be set.

Why is this needed:

UWP parity.

Resources

• https://xamlbrewer.wordpress.com/2017/02/22/using-a-dynamic-system-accent-color-in-uwp/

For which Platform:

• [ ] iOS
• [x] Android
• [ ] WebAssembly
• [ ] WebAssembly renderers for Xamarin.Forms
• [ ] macOS
• [ ] Skia
  • [ ] WPF
  • [ ] GTK (Linux)
  • [ ] Tizen
• [ ] Windows
• [ ] Build tasks
• [ ] Solution Templates

[MartinZikmund]

Gotchas:

• The accent color can potentially be anything (its "rules" are not clearly defined). We need to ensure that we don't use it as the "default accent" color, in case it would break accessibility (or adjust it by shifting the color to a safe range)
• It will require adding some color helpers for RGB -> HSL conversion, so the color brightness can be changed properly to scale to darker/lighter

[davidjohnoliver]

Note that (according to UWP docs at least, I haven't tested) the base accent color can also be overridden by app code:

<Application.Resources>
    <ResourceDictionary>
        <Color x:Key="SystemAccentColor">#107C10</Color>
    </ResourceDictionary>
</Application.Resources>

[MartinZikmund]

@davidjohnoliver we have that tracked in #3555 , that would be even more useful to support 👍 (I think most apps override accent color anyway). However, even if overwritten in resources, UISettings should still return the "system" color.

[jeromelaban]

Browsers also support this feature now: https://github.com/w3c/csswg-drafts/issues/5187#issuecomment-800651916

[jeromelaban]

Now supported in major browsers: https://twitter.com/una/status/1506383666548822020

Twitter

una.css 👩🏻‍💻🇺🇦 on Twitter

“`accent-color` is now supported in all modern browsers 🎉🎨

One line of code enables you to style previously-hard-to-access form controls like checkboxes and radio buttons

body { accent-color: hotpink; }

Works really nicely with the color-scheme property too! https://t.co/YRFQYyKQ2G”

[kucint]

@MartinZikmund: Uno currently only uses the default "windows" blue, and is ignoring the windows "accent-color", see #16608 Can this be fixed?