Ensure that we use the current user context when adding nav mode listeners

- Cache the current user context so we don't recreate it each time
- Also ensure that we always update the enabled state regardless of which
  user SysUI was started in (ie. restarting sysui while running as a
  non-primary user)

Bug: 132644959
Test: Kill sysui for secondary user, ensure it does not pick up primary
      user's nav mode

Change-Id: Iedbf56c18bf0901bf96ccccfe4d7b8c46161280b