Remove opaque-on-interaction from nav bar.

And decouple it from the status bar opaque-on-interaction logic.

It's still important to track nav bar interaction for hideybar
suspension purposes.

Also fix a sysui NPE that can occur when restarting SystemUI
(vs the shell).

Bug:10606136
Change-Id: I66a15e02cff352e26b25aebc1c42fb58c042effa