Fix drag drop accessibility

[danielrhammond]

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?)