Fix notification_policy user_set upgrade error

Bug: 203689334
Test: atest ManagedServicesTest, NotificationAssistantsTest
Test: manual test: upgrade from R(RP3A) to this and Q(QQ3A) to this with 1) User didn't change NAS, 2) User changed to new default NAS, 3) User changed to different NAS, 4) user changed to None
Change-Id: I64aa210b56d6350dea27b1b83c160ae147eb3a28