Lazily create system reserved channels

To reduce memory consumption (permanent) and boot time (first time after
the flag is turned on), only create channels when they are immediately
needed.

Test: NotificationManagerServiceTest
Test: PreferencesHelperTest
Test: NotificationAssisistantServiceTest
Test: NotificationManagerTest
Flag: android.service.notification.notification_classification
Fixes: 374186404
Change-Id: I63e9e93283d78c4e98906cccb6638e95d9206a0a