Enhancement: Search: Xamarin.Forms - show list highlight when individual results are displayed

[nCastle1]

When individual results are selected and the result list view is being displayed, UWP and WPF native implementations show the result with a distinct style.

The current Xamarin.Forms implementation doesn't visually identify the selected item, aside from the callout display.

Resolving this issue requires two changes:

• Update PART_ResultView_ItemSelected to only set the selected item to null if EnableIndividualResultDisplay is false, or there is only a single result.
• Create a custom viewcell and viewcell renderer (with implementations for iOS, Android, and UWP) that supports a selection color. The default ViewCell implementation works well with selection on UWP, but isn't visually distinguishable on iOS.