As far as I can see from API docs, the screen brightness is not protected by that forbidden credential. I'm not exactly sure what all power saving mode does. The one thing that can't be accessed (I think) without the credential is turning off WiFI scanning that it does normally. And also with power saving mode it only turns on WiFI connection when accessing internet, and switches it off after a while when net connection is not used anymore. These might not be settable in any other way, but I'm not sure.