Freeform workspace fixes

- Fixing issue with the wrong task view index being calculated for
  freeform workspace tasks causing them to be invisible when dragged
  to the stack
- Reducing unnecessary detach/reattach calls for freeform tasks
- Adding freeform workspace background to match exit to home animation

Change-Id: I8c00aba377601da92195ef301ba5da1ffb0045c5