Update reorder and folder creation radii

- Before, dragging anywhere in the cell (including space between cells) would trigger a reorder. Now, we compute the radius to the closest cell edge
- For creating folders on icons, we split the difference between the reorder circle (above) and the icon size

Test: enable DEBUG_VISUALIZE_OCCUPIED and manually drag to ensure works as described above
Bug: 204406063
Change-Id: I368714634ed42a930fd16849f2bde1589df1aa63