rotation: added code to check visibility of given ec before setting rotation in _e_cl...

rotation: added code to check visibility of given ec before setting rotation in _e_client_rotation_zone_set

we need to check ec is visible before rotating it.

e.g. camera -> image-viewer launching case

  image-viewer: bg_ec is set to image-viewer ec at the previous phase of this loop
  ec: camera

  if we decide to rotate camera ec which is obscured by image-viewer window,
  then camera app will not send rotation done event. thus it occurrs rotation time
  out error. to resolve this issue we should exclude obscured camera ec from
  rotation list.

Change-Id: Icd531284f6a894fea61d2e4f283a858c8ca69d83