Settings: don't remove navigation bar prefs when toggling

For devices which support the KeyDisabler class, toggling the nav bar
removes the navigation bar category, but does not add it back. If a
device can have the nav bar toggled, don't remove the category, but just
disable/enable it.

[mikeioannina]: Enable categories that depend on navbar toggle
only when the navbar switch has finished.

Change-Id: Ie527e3eb357e4a46097257d90c07c78e61f043a9
Signed-off-by: Roman Birg <roman@cyngn.com>