Investigate quicker lookup of nested horizontal ScrollView on Android

For bi-directional scrolling, on Android we nest a horizontal ScrollView in a vertical one. For every touch, we have to potentially feed that touch info to the horizontal ScrollView. The lookup for the nested ScrollView uses the `GetChildrenOfType` extension method. But there's only ever one nested ScrollView - we could probably avoid the loop through the ScrollView children (and the enumerable) by simply tagging the nested ScrollView with a known Id.