Settings: check if the icon choser app give valid content back

due that a lot weird filemanager exists which show up in the action
choser to be able to crop even if they do not (and a lot famous
one do that as well) check if the file which returns exists and
if it has content at all. Otherwise inform the user that
the chosen app does not support this action

Change-Id: I66524c4b07714944bea4d3e264d918d0b8b662ff