Correct me if I'm wrong, but right now the semantics are as follows: If the user manually calls `setUIAcls`, the UI ignores `spark.ui.acls.enable` even if it's false. This is a little confusing to me. It seems that `spark.ui.acls.enable` should be the ultimate stopper for whether ACLs should be enabled for the UI at all. If this is set to false, then we should obey it.

This is the semantics of `spark.eventLog.enabled`, for example. There's no other way to enable event logging other than setting the flag. I think the user may be a little confused if they explicitly set `spark.ui.acls.enable` to false but somehow there are still ACLs on their UIs.