Move the AllocationToken entity to the token subpackage

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=204485366

java/google/registry/model/domain/token/AllocationToken.java

@@ -0,0 +1,96 @@
+// Copyright 2018 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.model.domain.token;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkState;
+import static google.registry.util.PreconditionsUtils.checkArgumentNotNull;
+
+import com.googlecode.objectify.Key;
+import com.googlecode.objectify.annotation.Entity;
+import com.googlecode.objectify.annotation.Id;
+import google.registry.model.BackupGroupRoot;
+import google.registry.model.Buildable;
+import google.registry.model.CreateAutoTimestamp;
+import google.registry.model.annotations.ReportedOn;
+import google.registry.model.reporting.HistoryEntry;
+import java.util.Optional;
+import org.joda.time.DateTime;
+
+/** An entity representing an allocation token. */
+@ReportedOn
+@Entity
+public class AllocationToken extends BackupGroupRoot implements Buildable {
+
+  /** The allocation token string. */
+  @Id String token;
+
+  /** The key of the history entry for which the token was used. Null if not yet used. */
+  Key<HistoryEntry> redemptionHistoryEntry;
+
+  /** When this token was created. */
+  CreateAutoTimestamp creationTime = CreateAutoTimestamp.create(null);
+
+  public String getToken() {
+    return token;
+  }
+
+  public Key<HistoryEntry> getRedemptionHistoryEntry() {
+    return redemptionHistoryEntry;
+  }
+
+  public boolean isRedeemed() {
+    return redemptionHistoryEntry != null;
+  }
+
+  public Optional<DateTime> getCreationTime() {
+    return Optional.ofNullable(creationTime.getTimestamp());
+  }
+
+  @Override
+  public Builder asBuilder() {
+    return new Builder(clone(this));
+  }
+
+  /** A builder for constructing {@link AllocationToken} objects, since they are immutable. */
+  public static class Builder extends Buildable.Builder<AllocationToken> {
+    public Builder() {}
+
+    private Builder(AllocationToken instance) {
+      super(instance);
+    }
+
+    public Builder setToken(String token) {
+      checkState(getInstance().token == null, "token can only be set once");
+      checkArgumentNotNull(token, "token must not be null");
+      checkArgument(!token.isEmpty(), "token must not be blank");
+      getInstance().token = token;
+      return this;
+    }
+
+    public Builder setRedemptionHistoryEntry(Key<HistoryEntry> redemptionHistoryEntry) {
+      getInstance().redemptionHistoryEntry =
+          checkArgumentNotNull(redemptionHistoryEntry, "redemptionHistoryEntry must not be null");
+      return this;
+    }
+
+    public Builder setCreationTime(DateTime creationTime) {
+      checkState(
+          getInstance().creationTime.getTimestamp() == null, "creationTime can only be set once");
+      getInstance().creationTime = CreateAutoTimestamp.create(creationTime);
+      return this;
+    }
+  }
+}