Xamarin.forms: [iOS] Consider using IndexPath.Item instead of IndexPath.Row in CollectionView implementation

[adrianknight89]

I've seen various places where accessing CollectionView items are done via the Row property, but the correct property to check is Item. For more information, see https://stackoverflow.com/questions/14765730/nsindexpath-item-vs-nsindexpath-row.

[hartez]

Waiting for #6590 to get merged (which will fix most of these), then I'll clean up the last 3 instances.