Fix misaligned folder creation drag over target

Previously, the folder creation distance was based on the center of the cell, rather than the *visual* center, i.e. around the icon. Updated to use existing getWorkspaceVisualDragBounds() to handle this.

Test: with DEBUG_VISUALIZE_OCCUPIED=true, ensure green circles are centered around the app icons; manually drag and drop to check the drawn regions are correct
Bug: 204406063
Change-Id: I691a5cbbfc18c88436b88b7bda42f7920b9a5750