Make CollectionView SelectedItem and SelectedItems binding function correctly (#6085)

Make CollectionView SelectedItem and SelectedItems binding function correctly (#6085)

* Add automated test for CollectionView single selection bound item

* Make SelectedItem Two-Way

* Multiple selection test page

* Bindable SelectedItems implementation

* Add automated test

* Simplify null checks

* Add Preserve attribute so linker doesn't break test

* Make multi-item select test smaller so it passes UITests on smaller screens

* Clearer list-to-string method

* Clear native selection on iOS when SelectedItem set to null

fixes #6158
fixes #5832