1/ Initialize assistant state change listeners once with the nav bar helper

- Consolidate the meaning of init/destroy into the
  registration/unregistration of the nav bar helper state callbacks.
  This allows us to track the case where we are replacing the bars
  to prevent churn in unregistering and reregistering shared callbacks
- Using the above, we can only register the assistant content observers
  when the first bar is registered, and keep it registered across
  bar replacements

Bug: 219035565
Test: atest NavBarHelperTests
Test: atest SystemUITests

Change-Id: I2444bb0b27086b72212ef05b8c975c19a05a35d5