Please implement `Windows.ApplicationModel.Background`

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:

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

Anything else we need to know?