Ignore nav bar alpha changes while user is not yet setup

- SUW can change the display configuration which causes the nav bar
  fragment to be recreated which adds a listener to OverviewProxyService
  which calls back this method with the last alpha (default 1).  When
  in 2/3 button, it's not an issue because the back button is visible
  anyways.  But since we switched to gesture nav during SUW with the
  artifical back button, this code was causing the home handle to be
  updated as well to alpha 1.

Bug: 161081668
Test: Start SUW > Change font size > verify bar doesn't show
Change-Id: If0271d78d319f89f25d4c51da29e6676e8218812