Keydisabler: fix button light restore logic

Do not assign default button brightness if backup key is not found.
Instead do nothing. This will prevent brightness being reset to default
on boot when navbar is disabled.

If backup key is not found when actually toggling navbar then we've got
bigger problems.

Change-Id: I649bf1480a42f2cbe94d0a47a0c6055c934b4f56