Special handling for intent coming from "welcome" notification.

This notification is an introduction to new notification permission changes in T and is shown to the user upon upgrade; this change records that the user has seen/interacted with the notification already so we don't have to keep showing it.

This change essentially makes a copy of the existing functionality of NotificationAppListActivity, but meant only for access internally (so that neither the activity nor the associated action is exported/publicly accessible).

Bug: 225373531
Test: manual with the change that sends the notification
Change-Id: I20c6084652ea11a8d0a002a21561fe50b9cf5de3