Enable upower to report correct percentage

This also calculates times-to-empty and time-to-charge, as opposed to Android
adaptations (they read only the "capacity" field)

upower takes the full approach:
https://github.com/mer-packages/upower/blob/master/upower/src/linux/up-device-supply.c#L695
other implementations take codepath at L704

Change-Id: Ia651275fe2c6df7fe7916b60b26427b2876478cc