Since serial property cannot change and has no NOTIFY function, mark it as constant, to avoid warnings when the property is used in qml expression.
For instance in kinfocenter energy_info kcm :
QQmlExpression: Expression file:///home/meven/kde/usr/share/kpackage/kcms/kcm_energyinfo/contents/ui/main.qml:452:29 depends on non-NOTIFYable properties: Solid::Battery::serial