luci-base: FlagValue fix type of enabled and disabled properties

The form.Flag checkbox allows to specify a value for enabled/disabled e.g. for a dropbear:

    o = s.option(form.Flag, 'PasswordAuth');
    o.enabled  = 'on';

So the property type should be a string but not just a number.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>