CMSettings: intercept Settings calls to DEV_FORCE_SHOW_NAVBAR

Because DEV_FORCE_SHOW_NAVBAR has lived in Settings.System,
Settings.Secure, and now in CMSettings.Secure, we need to return the
proper value no matter which place an app might query.

Ticket: CYNGNOS-2480

Change-Id: Ie84df9763aa3714ec4ce4d033dc73be4de3f1f00
Signed-off-by: Roman Birg <roman@cyngn.com>
(cherry picked from commit fd4b383b)