if you got an HI12 and the brightness slide bar just do nothing on Android.
I found that installing any free app to control brightness and disabling "auto level" in the app just make the slider bar working.
You can then remove the brightness app you add.
Also - here is a permanent fix for the Android brightness issue - you can just flash this .zip file and you will be able to lower the brightness in Andoid down very low (via the normal brightness slider). This works for the Hi10 Pro, Hi10 Plus and Hi12 (as well as most other devices)....