power: Stop making time estimates

Any time estimates we can come up with are notoriously unreliable; even on devices that report correct (dis)charging rates, any change in workload, screen brightness etc. can throw our estimate off. So instead, limit ourselves to only show the current percentage and leave its interpretation to the user. As an added bonus, we end up with shorter strings that are less likely to cause problems with ellipsization when translated.

https://bugzilla.gnome.org/show_bug.cgi?id=708472

Fixes #2256