Clean mPreviousSolution on change of CellLayout target

The previous reorder solution is not peropperly cleaned and
that leads to wrong solution being used. For example when
longpressing an icon, it triggers a drag but the drag never
finishes.

Fix: 261122618
Test: You can no longer overlap a shortcut in the same app icon (see attached bug)
Test: atest ReorderWidget

Change-Id: Iff8651926cc4179561761c7ce0ac5007f13fc9af