Fix crash when dragging widget to remove in hub edit mode

It happens when the last item is taller than the dragged item, as we
add a Spacer at the end of the list when drag starts and save its
initial index. This index becomes invalid (out of bound) after the dragged widget is removed.
The fix is to check if the last item is a Spacer item on drag ends and remove it if so.

Fixes: 415898439
Test: on device
Flag: EXEMPT bugfix
Change-Id: I188cf0454df955830b5b830fcc77e5aa7fef87d2