backlight: Keyboard backlight toggle on to last set value
Keyboard back light toggle in quick settings flips keyboard brightness to 0 or 100.
If a user sets an intermediate level, and then toggles off and on, this resets it to the maximum instead of the last set value. The user expectation would be to toggle back to the last set value.
The new toggle function restores the brightness to the last set value prior to toggling off, during the current session.
Closes: #7873