Initialize instance_id at enqueue rather than post.

Because assistant adjustments were potentially happening at enqueue time but we weren't instantiating instance IDs until postNotification, all of the notification classification atoms were getting logged with instance ID 0.

Fixes: 417536505
Test: atom tester, NotificationManagerServiceTest
Flag: EXEMPT bugfix
Change-Id: I75f8c635d01eea4fe39764592478d525c30cad3c