Unable to specify PROPERTY_SPECIAL_USE_FGS_SUBTYPE via Service Attribute about xamarin-android

[jonpryor]

Related: https://developer.android.com/guide/topics/manifest/property-element

We should probably introduce a PropertyAttribute, which emits <property/> within the appropriate scope:

[AttributeUsage (AttributeTargets.Class, AllowMultiple=true)]
public partial class PropertyAttribute : Attribute {
    public PropertyAttribute (string name);
    public string Resource {get; set;}
    public string Value {get; set;}
}

which would allow:

[Service]
[Property ("android.app.PROPERTY_SPECIAL_USE_FGS_SUBTYPE", Value="Explanation")]
partial class FocusSessionService : Service {
}

Open design question: from the <property/> docs:

Note: A <property> tag should not contain both android:value and android:resource attributes. If that happens, android:resource is preferred.

Given this, it might instead be better to have PropertyResourceAttribute and PropertyValueAttribute instead of a single PropertyAttribute.