Ensure GroupValueChangedMessage is received

[cshung]

Description of Change

There are three fixes in this change.

1. The message registration type is changed from ILayout to Element. This is because _layout in RadioButtonGroupController is actually an instance of Element.

2. The checking for Value != args.Value is changed to !Value.Equals(args.Value). This is because Value and args.Value can come from different sources and object reference equality often doesn't work.

3. In case the SelectedValue of the radio group is changed before the radio buttons are added to the group, the radio button missed the event to set its default value and won't set its IsChecked state. Therefore we need to make sure when a radio button is added to the group, it sets its IsChecked state to the correct value.

Issues Fixed

Fixes #5645

@softlion

[rmarinho]

@cshung you mind rebase on main, to see if all tests pass. Thanks