Allow the NAS to set a type for notifications.

Some of the types will redirect a notification to a new, reserved,
silent-by-default channel.

Fixes: 344013362
Fixes: 344015982
Fixes: 344015984

Test: NotificationAssistantServiceTest
Test: NotificationRecordTest
Test: PreferencesHelperTest
Change-Id: I1173dd9600164621b2e9ce08527d90a3177297d2