Settings: fix button back light restore logic

Only persist backlight data if it's modified by the user, otherwise we
can just restore the default framework value when enabling.

Always restore a backlight value setting, either the framework default,
or user saved value.

Change-Id: I2d6e95a48bc4ac4dbe6d639d87dc93ec1799c290
Signed-off-by: Roman Birg <roman@cyngn.com>