applets/batterymonitor: Set brightness slider minimum to 0 for keyboard and 1 for screen

The rationale for setting the minimum to 1 instead of 0 was that a user might inadvertently turn off their screen and be unable to revert it; but this is not an issue for keyboard backlight and only leads to inconsistent behavior (see powerdevil!206 (comment 735640)), so only apply that cap to the screen brightness slider and then do it irrespectively of the number of brightness steps.