Re-add BatteryInfo qml

The battery declarative class was never reinstated after the battery
class for refactored.

Also fix the upower backend to work correctly.

Task-number: QTBUG-42958
Change-Id: I4f627426f6c716a5317eb28ed85fd9da793f1940
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>