SystemUI: hide up arrow NavRing indicator if no targets present

If there aren't any navring targets setup (the user has removed all of
them, or maybe search assistant is not present) then there is still a
hint in the lockscreen to pull up from the nav bar.

If there are no targets setup, hide the arrow indicator.

Change-Id: I65103a4cec668a9a8b3f678f991f5acba17d19f5
Signed-off-by: Roman Birg <roman@cyngn.com>