Use random ids when creating conversation channels for a package

This ensures we don't exceed the 1000-character limit (which could even end up replacing another conversation channel with a common prefix).

Bug: 432250872
Test: atest NotificationManagerServiceTest
Flag: com.android.server.notification.random_conversation_ids
Change-Id: Ifdebb94e8ba4bcadf8e153c0bbbccb0db0fd7797