Bug 971469 - [System2] Instantiable BatteryOverlay

https://bugzilla.mozilla.org/show_bug.cgi?id=971469

We can not assign BatteryManager as a constructor as it is already on the window object: https://developer.mozilla.org/en-US/docs/Web/API/BatteryManager

Because of this, the patch renames BatteryManager to BatteryOverlay.