Fix notifications triggering too often.

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

@@ -305,6 +305,7 @@ public class MessageNotifier {
     builder.setContentIntent(notifications.get(0).getPendingIntent(context));
     builder.setGroup(NOTIFICATION_GROUP);
     builder.setDeleteIntent(notificationState.getDeleteIntent(context));
+    builder.setOnlyAlertOnce(!signal);
 
     long timestamp = notifications.get(0).getTimestamp();
     if (timestamp != 0) builder.setWhen(timestamp);
@@ -347,6 +348,7 @@ public class MessageNotifier {
     builder.setMostRecentSender(notifications.get(0).getIndividualRecipient());
     builder.setGroup(NOTIFICATION_GROUP);
     builder.setDeleteIntent(notificationState.getDeleteIntent(context));
+    builder.setOnlyAlertOnce(!signal);
 
     long timestamp = notifications.get(0).getTimestamp();
     if (timestamp != 0) builder.setWhen(timestamp);