Fix regression in SysUI boot time

- If we defer creating the fragment until the view is attached, that
  seems to throw off the flow of sysui startup (maybe because the actual
  nav bar fragment isn't set until later).  Instead, pull out the
  attached listener and clear the fragment ref after the view is attached
  so that we still release the fragment ref.

Bug: 162575036
Test: Kill sysui multiple times and check sysui boot times
Test: Switch navigation modes several times and verify there's still only
      one ref to nav bar fragment & view
Change-Id: I636837771e8011747de79c7609443ef562804eea