[SB][Screen chips] Move `ongoingCallNotification` flow to background.

Fixes: 357170372
Bug: 332662551
Bug: 328584859

Flag: com.android.systemui.status_bar_use_repos_for_call_chip
Test: Start ongoing call -> verify chip still shows
Test: atest ActiveNotificationsInteractorTest
OngoingCallControllerViaRepoTest

Change-Id: I63ab03925d361b641dcd096d9b46548c489b7aaa