Please implement `Windows.ApplicationModel.Background`

[pkar70]

What would you like to be added:

Simple timer app activation, i.e. var builder = new Windows.ApplicationModel.Background.BackgroundTaskBuilder(); builder.SetTrigger(new Windows.ApplicationModel.Background.TimeTrigger(int, bool)); builder.Register()

Why is this needed:

Many apps should be activated once for a while (e.g. once per hour), to make something. I want to convert to Uno (and Android) app which checks air pollution and show notification about current status.

For which Platform:

• [X] iOS
• [X] Android
• [ ] WebAssembly
• [ ] WebAssembly renderers for Xamarin.Forms
• [ ] Windows
• [ ] Build tasks
• [ ] Solution Templates

Anything else we need to know?

[pkar70]

This is implemented in PR#2655 (for Android).