batterymonitor: fix repeated low battery warnings

[Wekendd]

Fixes issue #736 (which was closed likely due to inactivity). There was a PR aiming at fixing this (#915) but it was closed for some reason. New behavior:

• Battery toasts only go off if the current battery level specifically drops to their level (old behavior used to send all toasts that satisfied specified value >= current battery level--this caused multiple toasts to go off at once)
• When charger unplugged or shell started, sends the closest battery warning toast above the current battery level (if one exists).
• No reliance on temporary state (not buggy across reloads)