settings: fix and pimp custom lockscreen handle (2/2)

problem on current implementation is that it is not
multiDpi compatible. As well it looks a bit weird if
the user set a square image always. This commit does

- introduce central circleimage method to get circle image crops
- before it was assumed that the height of the handle image is
  108px (mdpi base) which is correct but the original handle
  has a lot unused space around to fit better into the view.
  Whatever after messurment of the real height 68px is correct
  of course calculated against the current density
- both above combined results into a always nice looking
  custom circle icon on the lockscreen dpi and device undependent
- saved file in settings calculates the needed image size
  base on the current density.

Change-Id: Iba46fdf725be0fe785234bda3fd1ff8dbb4c1467