Write identity key into 'account' object.

This is the beginning of a migration to storing one identity
key per account, instead of the braindead duplication we're
doing now.  Part one of a two-part deployment in the schema
migration process.

src/main/java/org/whispersystems/textsecuregcm/entities/PreKeyList.java

@@ -17,6 +17,7 @@
 package org.whispersystems.textsecuregcm.entities;
 
 import com.fasterxml.jackson.annotation.JsonProperty;
+import com.google.common.annotations.VisibleForTesting;
 import org.hibernate.validator.constraints.NotEmpty;
 
 import javax.validation.Valid;
@@ -27,6 +28,7 @@ public class PreKeyList {
 
   @JsonProperty
   @NotNull
+  @Valid
   private PreKey lastResortKey;
 
   @JsonProperty
@@ -38,7 +40,17 @@ public class PreKeyList {
     return keys;
   }
 
+  @VisibleForTesting
+  public void setKeys(List<PreKey> keys) {
+    this.keys = keys;
+  }
+
   public PreKey getLastResortKey() {
     return lastResortKey;
   }
+
+  @VisibleForTesting
+  public void setLastResortKey(PreKey lastResortKey) {
+    this.lastResortKey = lastResortKey;
+  }
 }