Add onlyModernNotificationClearance feature flag.

Signal/src/UserInterface/Notifications/UserNotificationsAdaptee.swift

@@ -180,9 +180,13 @@ extension UserNotificationPresenterAdaptee: NotificationPresenterAdaptee {
 
     func clearAllNotifications() {
         AssertIsOnMainThread()
+
         notificationCenter.removeAllPendingNotificationRequests()
         notificationCenter.removeAllDeliveredNotifications()
-        LegacyNotificationPresenterAdaptee.clearExistingNotifications()
+
+        if !FeatureFlags.onlyModernNotificationClearance {
+            LegacyNotificationPresenterAdaptee.clearExistingNotifications()
+        }
     }
 
     func shouldPresentNotification(category: AppNotificationCategory, userInfo: [AnyHashable: Any]) -> Bool {

SignalServiceKit/src/Util/FeatureFlags.swift

@@ -72,7 +72,13 @@ public class FeatureFlags: NSObject {
     @objc
     public static let strictSyncTranscriptTimestamps = false
 
+    private static let isPrivateBeta = false
+
     // Don't enable this flag in production.
     @objc
-    public static let strictYDBExtensions = false
+    public static let strictYDBExtensions = isPrivateBeta
+
+    // Don't enable this flag in production.
+    @objc
+    public static let onlyModernNotificationClearance = isPrivateBeta
 }