Fix bug with drag visualization and UserFolders.

When dragging an app shortcut, it was possible that we'd show a red rectangle
around a cell occupied by a UserFolder. This shouldn't be possible -- as soon
as that cell becomes the target drop cell, the folder should start handling
the drag and drop events.

Change-Id: I1b7a8b1aa9aeb7e2f1bd51ce8d947c06455e988f
7 files changed