Fix drag drop accessibility

From internal comment/bug:

> iOS assigns `accessibilityDragSourceDescriptors` and `accessibilityDropPointDescriptors` to the cell, but doesn't look up the hierarchy from elements to their ancestors to see if drag and drop applies, If this cell were directly `isAccessibilityElement = YES` then this would not be a problem, but since we're hosting something else which (may be) an accessibility element, we need to find it and copy over the location descriptors so drag&drop applies.

So CollectionViewHostCell needs to set these properties on its host view (and maybe on the child View itself too?)