Adds flag for UI changes in bundles/classification

Currently the notification bundles/classification flag is guarding new
API that needs to be rolled out in advance of UI changes, so we create a
new flag to guard the UI changes separately.

Bug: 374791469
Test: EXEMPT new flag def
Flag: android.app.notification_classification_ui
Change-Id: Ic34121ac33560521312d73853eda6220fa9d05e9