Streamline transfer resolving helpers in ResourceFlowUtils

In the great flow flattening, ResourceFlowUtils grew a couple nice helpers
for rebuilding transferrable resources (Domains and Contacts) upon the
resolution of a transfer - approvePendingTransfer() and denyPendingTransfer().

Most transfer-resolving callsites use one of these two helpers, but for legacy
reasons the deletion flows (DomainDeleteFlow and DeleteContactsAndHostsAction)
were instead using the "manual" resolvePendingTransfer() method or its even more
low-level createResolvedTransferData() helper instead of denyPendingTransfer().
It's simpler to just have two options - approve and deny - so this CL inlines
createResolvedTransferData() into resolvePendingTransfer() and makes the latter
a private helper for the approve/denyPendingTransfer() public helpers.

This CL also adds sanity checks that approve/denyPendingTransfer() are called
only with the logically appropriate values of TransferStatus.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170819358

java/google/registry/model/transfer/TransferStatus.java

@@ -54,4 +54,10 @@ public enum TransferStatus {
   public boolean isApproved() {
     return this.equals(CLIENT_APPROVED) || this.equals(SERVER_APPROVED);
   }
+
+  public boolean isDenied() {
+    return this.equals(CLIENT_CANCELLED)
+        || this.equals(CLIENT_REJECTED)
+        || this.equals(SERVER_CANCELLED);
+  }
 }