Annotate NotificationManagerInternal with @Keep

This prevents removal of the class upon build time for other system
services to use.

Bug: 283030212
Test: build
Flag: EXEMPT trivial
Change-Id: I272beb6791fe081ace857b9819d0237b7698ede1
diff --git a/services/core/java/com/android/server/notification/NotificationManagerInternal.java b/services/core/java/com/android/server/notification/NotificationManagerInternal.java
index d5d4070..52ddb80 100644
--- a/services/core/java/com/android/server/notification/NotificationManagerInternal.java
+++ b/services/core/java/com/android/server/notification/NotificationManagerInternal.java
@@ -22,8 +22,11 @@
 import android.app.backup.BackupRestoreEventLogger;
 import android.service.notification.DeviceEffectsApplier;
 
+import com.android.internal.annotations.Keep;
+
 import java.util.Set;
 
+@Keep
 public interface NotificationManagerInternal {
     NotificationChannel getNotificationChannel(String pkg, int uid, String channelId);
     NotificationChannelGroup getNotificationChannelGroup(String pkg, int uid, String channelId);