Replace 'host.getSubordinateHost() != null' with 'host.isSubordinate()'

This is a cleanup in preparation for the next change that does a lot
of work with subordinate hosts, to make it easier to reason about in
complex code.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146689904

java/google/registry/model/host/HostResource.java

@@ -96,6 +96,10 @@ public class HostResource extends EppResource implements ForeignKeyedEppResource
     return superordinateDomain;
   }
 
+  public boolean isSubordinate() {
+    return superordinateDomain != null;
+  }
+
   public ImmutableSet<InetAddress> getInetAddresses() {
     return nullToEmptyImmutableCopy(inetAddresses);
   }