Allow the NAS to update channels and add @SystemApi to...

Allow the NAS to update channels and add @SystemApi to NotificationChannel#setImportantConversation.

This is to support the new conversation notification channel management feature.

Design doc: go/nas-25q2-blue

Bug: 374268500
Bug: 373599715
Change-Id: I044bf6800d092ec59be0d664912c3358b3dba60f
Flag: android.service.notification.notification_conversation_channel_management
API-Coverage-Bug: 374688623