Add New SecureSettings Constant for Notification Minimalism

Add a new SecureSettings constant for Notification Minimalism
and use it for LockScreenMinimalismCoordinator to replace
usage of lock_screen_show_only_unseen_notifications.

Bug: 330387368
Bug: 354047572
Flag: com.android.server.notification.notification_minimalism
Test: adb shell settings put secure
lock_screen_notification_minimalism <1|0>

Change-Id: Iecaf6d405201e1da8507b5300e5e77ae7d62da8b