As an exported debug apk on device it doesn't seem to work. Does it work for you like that?
I cannot export to apk at the moment. So i tested only in browser.
If Browser.isOnLine doesn't work in apks. I would consider that as a bug. It's not mentioned that it doesn't work. I would make a bug-report.
Have you tried to use PlatformInfo.ConnectionTyp? it should return 'none' if it's not conneted, I think.