Remove notification channels from history when they're deleted.

To make this possible, this change adds functionality for notification channel removal to NotificationHistory, NotificationHistoryDatabase, and NotificationHistoryManager and corresponding tests.

This change does not remove the notifications from deleted channels from the "recently dismissed" list (stored in mArchive); will follow up with that later.

Test: atest NotificationHistoryTest, NotificationHistoryDatabaseTest, NotificationManagerServiceTest; manual via creating channels, deleting them and verifying
Bug: 169349809

Change-Id: I7b1c137e516d14703665750f06554ac7ca44c90e