Improve handling of lastSubordinateChange and beef up tests

See [] for the comments that led to this.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=147796087

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

@@ -85,8 +85,11 @@ public class HostResource extends EppResource implements ForeignKeyedEppResource
   DateTime lastTransferTime;
 
   /**
-   * The most recent time that the superordinate domain was changed, or null if this host is
-   * external.
+   * The most recent time that the {@link #superordinateDomain} field was changed.
+   *
+   * <p>This should be updated whenever the superordinate domain changes, including when it is set
+   * to null. This field will be null for new hosts that have never experienced a change of
+   * superordinate domain.
    */
   DateTime lastSuperordinateChange;