KeyboardBrightnessController: Drop isValid() check for upower interface
isValid() seems to be racy when upower is just starting up (because we DBus-activated it earlier in BatteryController).
This causes us to think keyboard brightness control is not available even when it actually is.
To address this always make a call to the interface. The call will block until the interface is ready to either give us the correct result or we know it's not available because we get a relevant DBus error.
As a bonus print a warning message if we get any unexpected errors