fix: Committer stuck in retry loop on shutdown (#1062)

Close the partition subscriber factory (which contains the client stubs) last. This allows the Committer to send a pending commit to the server on shutdown.

Author: tmdiep

google-cloud-pubsublite/src/main/java/com/google/cloud/pubsublite/cloudpubsub/SubscriberSettings.java

@@ -315,14 +315,14 @@ Subscriber instantiate() throws ApiException {
     }
 
     List<ApiService> services = new ArrayList<>();
-    services.add(autoCloseableAsApiService(partitionSubscriberFactory));
     for (Partition partition : partitions()) {
       try {
         services.add(partitionSubscriberFactory.newSubscriber(partition));
       } catch (CheckedApiException e) {
         throw e.underlying;
       }
     }
+    services.add(autoCloseableAsApiService(partitionSubscriberFactory));
     return MultiPartitionSubscriber.of(services);
   }
 }