Fix double notifications in O+.

src/org/thoughtcrime/securesms/notifications/MessageNotifier.java

@@ -33,6 +33,7 @@ import android.os.AsyncTask;
 import android.os.Build;
 import android.service.notification.StatusBarNotification;
 import android.support.annotation.NonNull;
+import android.support.v4.app.NotificationCompat;
 import android.support.v4.app.NotificationManagerCompat;
 import android.text.TextUtils;
 import org.thoughtcrime.securesms.logging.Log;
@@ -306,6 +307,7 @@ public class MessageNotifier {
     builder.setGroup(NOTIFICATION_GROUP);
     builder.setDeleteIntent(notificationState.getDeleteIntent(context));
     builder.setOnlyAlertOnce(!signal);
+    builder.setGroupAlertBehavior(NotificationCompat.GROUP_ALERT_SUMMARY);
 
     long timestamp = notifications.get(0).getTimestamp();
     if (timestamp != 0) builder.setWhen(timestamp);
@@ -349,6 +351,7 @@ public class MessageNotifier {
     builder.setGroup(NOTIFICATION_GROUP);
     builder.setDeleteIntent(notificationState.getDeleteIntent(context));
     builder.setOnlyAlertOnce(!signal);
+    builder.setGroupAlertBehavior(NotificationCompat.GROUP_ALERT_SUMMARY);
 
     long timestamp = notifications.get(0).getTimestamp();
     if (timestamp != 0) builder.setWhen(timestamp);