Simplify immersive mode confirmation cling logic.

Instead of keeping track of confirmations per-package, track
a global confirmation per-user.  If the panic signal is received,
reshow the cling at most once per-user per-reboot.

Ensure the nav bar becomes visible after the panic signal.
Usually this happens as a side effect of showing the keyguard.
However, in the case where there is no keyguard (Security = None)
show the transient nav bar temporarily as a hint.

Also listen to the correct observer uri to pick up confirmation
setting changes.

Bug:12242125
Change-Id: Ic95e2a8630ec3802b8ef462fcaa92366b9343a3f