power: Label the PENDING_CHARGE state as "Not Charging"
The pending-charge state means AC power is on but the battery is not being charged. This can happen because its charge is above a certain threshold, to avoid short charging cycles and prolong the battery's life, or because the PSU is not powerful enough to charge the batteries.
Instead of lying to the user about something being estimated, we should simply tell the truth and set the label to "Not Charging".
Closes: #701 (closed).