Fix a couple small issues in nav bar fragment

- Remove attached state change listener when the view is detached
- Fix for hypothetical race, if the system calls setWindowState before
  the fragment view is created, we will drop the new window state and
  also not update the sysui state flags.  Instead, always update the
  window state and only update the nav bar window visibility if it
  exists or when it is created (which is already being done).

Bug: 147890990
Test: manual