• Fix issue #18 CurrentItem is not correct when SnapStyle = Center (iOS and Android)
• Consistency between CurrentItem update and snapping between iOS and Android

Snap center behavior changes

I had to make a choice with CurrentItem update when ```SnapStyle = Center````. As for this version, when scrolling to the first item or the last item, snap effect is deactivated in order to fully see the item.

Snap behavior and CurrentItem are updated accordingly:

• if first item is completely visible snap is deactivated and CurrentItem = 0
• if last item is completely visible snap is deactivated and CurrentItem = index-of-last-item