Make battery serial property constant

Summary:
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

Reviewers: Frameworks, davidedmundson

Reviewed By: davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23598