Enable Show Only Unseen Notifs when Minimalism is enabled

Enable the lock_screen_show_only_unseen_notifications feature
when the notification_minimalism flag is enabled. This change
makes it possible to turn on both features at the same time.

Bug: 330387368
Bug: 354047572
Flag: com.android.server.notification.notification_minimalism
Test: atest SystemUITests

Change-Id: I6ccdf93345fccb5530cbc9be807b6a67e26290b2