restore previous brightness value after toggling keyboard backlight

... provided it was not already zero, instead of always setting to maximum brightness.

How to test:

1. Have a keyboard with multiple brightness levels
2. Assign a shortcut for "Toggle Keyboard Backlight"
3. Set keyboard backlight to 50%, eg with the slider in the battery applet
4. Press toggle shortcut (to turn off)
5. Press toggle shortcut (to turn on again)

Before:
Keyboard brightness is 100%

After:
Keyboard brightness is 50%

BUG: 464755