Update shellcheck and shfmt to the latest version.
Fixes <https://github.com/koreader/koreader/issues/5152>.
Btw, you can apply shellcheck suggestions with a command like:
```
shellcheck --include=SC2250 -f diff *.sh | git apply
```
* Use getCapacityHW() to ensure latest battery capacity can be retrieved
* BackgroundRunner
* Start background_runner_spec.lua
* AutofrontLight plugin now uses BackgroundRunner plugin