core: add REPOWERD_BACKLIGHT_BACKEND environment variable
This is supposed to let porters select sysfs-based backlight backend via repowerd service override on devices where Android lights HAL is available, but does not handle screen brightness value in a sane (same as older devices) way.
Needed for OnePlus 6, Redmi Note 8(T), Poco M3 and possibly other devices.
Edited by Ratchanan Srirattanamet