Add a new API to create conversation notification channels from the NAS.

This API is guarded by a new flag, FLAG_NOTIFICATION_CONVERSATION_CHANNEL_MANAGEMENT.

Design doc: go/nas-25q2-blue

Bug: 374245127
Bug: 373599715
Change-Id: I79b4a864ebf044baf0912b7eef992c071cd4c573
Test: NotificationManagerServiceTest
Flag: android.service.notification.notification_conversation_channel_management