APNS: include a collapse-id for non-VOIP notifications

This has two benefits:

- The APNS server should only send an iOS client a single push
  notification for any missed messages while the device is offline
  (server-side coalescing). Note that the client can still turn that
  into multiple "user notifications" as it pulls from its queue.

- If multiple notifications get delivered but iOS is unable to process
  them (say, because the phone just restarted and hasn't been unlocked
  yet), the user should only get one "You may have received messages"
  notification (client-side coalescing).

service/src/main/java/org/whispersystems/textsecuregcm/push/APNSender.java

@@ -82,7 +82,8 @@ public class APNSender implements Managed {
     ListenableFuture<ApnResult> future = apnsClient.send(message.getApnId(), topic,
                                                          message.getMessage(),
                                                          Instant.ofEpochMilli(message.getExpirationTime()),
-                                                         message.isVoip());
+                                                         message.isVoip(),
+                                                         message.getCollapseId());
 
     Futures.addCallback(future, new FutureCallback<>() {
       @Override

service/src/main/java/org/whispersystems/textsecuregcm/push/ApnMessage.java

@@ -66,6 +66,14 @@ public class ApnMessage {
     }
   }
 
+  @Nullable
+  public String getCollapseId() {
+    if (type == Type.NOTIFICATION && !isVoip) {
+      return "incoming-message";
+    }
+    return null;
+  }
+
   @VisibleForTesting
   public Optional<String> getChallengeData() {
     return challengeData;

service/src/main/java/org/whispersystems/textsecuregcm/push/RetryingApnsClient.java

@@ -67,9 +67,9 @@ public class RetryingApnsClient {
     this.apnsClient = apnsClient;
   }
 
-  ListenableFuture<ApnResult> send(final String apnId, final String topic, final String payload, final Instant expiration, final boolean isVoip) {
+  ListenableFuture<ApnResult> send(final String apnId, final String topic, final String payload, final Instant expiration, final boolean isVoip, final String collapseId) {
     SettableFuture<ApnResult>  result       = SettableFuture.create();
-    SimpleApnsPushNotification notification = new SimpleApnsPushNotification(apnId, topic, payload, expiration, DeliveryPriority.IMMEDIATE, isVoip ? PushType.VOIP : PushType.ALERT);
+    SimpleApnsPushNotification notification = new SimpleApnsPushNotification(apnId, topic, payload, expiration, DeliveryPriority.IMMEDIATE, isVoip ? PushType.VOIP : PushType.ALERT, collapseId);
         
     apnsClient.sendNotification(notification).whenComplete(new ResponseHandler(result));