3/ Move wallpaper visibility listener to nav bar helper

- The bar transitions update based on the wallpaper visibility but this
  can be expensive to register/unregister frequently.  Instead, pull the
  listener out to NavBarHelper and keep it registered across the bars
  (it's a no-op for taskbar currently)
- We don't need the weak reference since nav bar helper is a singleton

Bug: 219035565
Test: atest NavBarHelperTests
Test: atest SystemUITests
Change-Id: I4b93437abd5e404981f119916557776090223cda