Export approximate battery levels that devices can use, exported by
the kernel as POWER_SUPPLY_CAPACITY_LEVEL_* values.
This avoids bizarrely accurate values showing up in UIs when we only
have ok/warning levels of accuracy in some cases.
https://bugs.freedesktop.org/show_bug.cgi?id=100359
There are multiple possible causes for the keyboard brightness to change,
e.g. SetBrightness may get called, or the brightness may get changed
through a hotkey which is handled in firmware.
https://bugs.freedesktop.org/show_bug.cgi?id=98404
