Don't store Launcher as a class variable in CellLayout

CellLayout can be used by non launcher context, e.g. style &
wallpaper preview. To be safe, don't store Launcher as class
variable.

Test: Preview in style & wallpaper works fine
      Drag icons from hotseat.
      Drag widgets from workspace.

Bug: b/182282587
Change-Id: I9358eebce66ff1a46adbfb0602ae2bc970a921a6