Settings: use correct default value for backlight brightness

When the user has enabled the nav bar through the setupwizard and
returns back to the buttons settings and toggles the nav bar off, the
backlight brightness would not be restored because there was no previous
value to read from, so use the default framework value if none exists.

Change-Id: If4ce90bcde3c9a59f26e49be2c585df844ead6bd
Signed-off-by: Roman Birg <roman@cyngn.com>