Drag n Drop not working in 4.6.1

[BillCarsonFr]

Fixes #1187

Drag N Drop support is broken in 4.6.1 (working in 4.5). Due to migration of EpoxyTouchHelperCallback from java to kotlin and missing @nullable annotation in ItemTouchHelper

The following can return null (checked in source code)

public ViewHolder chooseDropTarget(@NonNull ViewHolder selected,
                @NonNull List<ViewHolder> dropTargets, int curX, int curY) 

And code calling this is checking for null

 ViewHolder target = mCallback.chooseDropTarget(viewHolder, swapTargets, x, y);
        if (target == null) {
            mSwapTargets.clear();
            mDistances.clear();
            return;
        }

So i simply made the kotlin code using optionals.

I also added a drag n drop activity in sample app to test it.
I notice also that StickyHeaderActivity was not working due to epoxy_view_stub taking all vertical space

[elihart]

Great! Thank you for the fix and the improved samples