Notification API hardening: add FLAG_SILENT

 Replace Notification.GROUP_KEY_SILENT with FLAG_SILENT.
 Prevents silent notifications to be forced grouped by GroupHelper.

Flag: android.service.notification.notification_silent_flag

Test: atest NotificationTest NotificationManagerServiceTest
Test: atest NotificationInterruptStateProviderImplTest
Test: atest VisualInterruptionDecisionProviderImplTest

Bug: 336488844
Change-Id: Icd0ae4d647223e481a1857f98b2732ccb52eddca