You should use
int previousScreenBrightness = android.provider.Settings.System.getInt(getContentResolver(),Settings.System.SCREEN_BRIGHTNESS);
to get system brightness value (same with the brightness value in Settings>Display>Brightness).(Range from 0 to 255)
What you get (-1.0) from WindowManager.LayoutParams indicates BRIGHTNESS_OVERRIDE_NONE, which means your current Activity brightness value is not override by WindowManager.LayoutParams attributes. When the value set to 0 to 1, then your current Activity brightness value would influenced by the setting.