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)

EDIT: Behavior changed below.

[Wekendd]

Made changes to make this work with the potential of percentage skips (most likely resulting from hibernation). Changes:

• Battery toasts only go off if the last percent was above their specified level, and current battery percent is now at or below their level
• Temporary state is reintroduced with a single variable lastPercentage. I couldn't think of any way to do this without using state. However, unlike the old solution (with a flag for each notification level), this implementation works, is clean across reloads/reboots, and only sends the lowest applicable notification.

[soramanew]

LGTM, thanks!