Add correct flags dependency for force-grouping tests

 The tests below should only run when FLAG_CHECK_AUTOGROUP_BEFORE_POST is also enabled:
 - testOnlyForceGroupIfNeeded_newNotification_wasAutogrouped
 - testOnlyForceGroupIfNeeded_newNotification_notAutogrouped


Flag: android.service.notification.notification_force_grouping

Change-Id: I810a6f74c91b480686a126b0727c5eb3e27bf916
Test: atest NotificationManagerServiceTest
Bug: 367511025