only recreate onboarding controller on view change

Flag: com.android.systemui.notification_bundle_ui
Fixes: 426424951
Test: manual
  1. Have bundle onboarding in shade
  2. Scroll down such that onboarding is barely visible above shelf
  3. Receive alerting notification
  Observe: onboarding not invisible in shade
Change-Id: I464412d509618d343084a29b13d2c585ff14c039