[applets/batterymonitor] Handle "plugged in but still discharging" state

Right now we handle the "plugged in but not charging" state but not the state where you're plugged in but the power supply doesn't supply enough power to prevent discharging anyway. Let's handle it.


(fake screenshot because I don't actually have hardware that can reproduce this use case)

cc @broulik

Edited by Nate Graham

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