Fix Zen tests setup in NotificationManagerServiceTest

ZenModeHelper can only act on Zen configurations for users that have been switched to at least once (or user 0). When running the unit test on an HSUM device this had never happened, so a couple tests were failing.

Fixes: 383071237
Fixes: 383072845
Test: atest NotificationManagerServiceTest
Flag: TEST_ONLY
Change-Id: Ie33d277435d8d3044ed4ea5a616145479ce4d67f