Fix panic gesture for hidden navigation bar

And make sure we don't show transient bar while notification shade is
controlling system UI.

Fix: 161514002
Test: 1. Open an immersive app (which hides navigation bar).
      2. Press power key twice within 5 seconds.
      3. Return to the app (unlock lockscreen if there is any).
Change-Id: I61ada33306f767acdd194de76ce6d15b4e61387f