Break circular dependency between core and util (#379)

* Break circular dependency between core and util

Created a new :common project and moved a minimum
number of classes to break the circular dependency
between the two projects. This gets rid of the
gradle lint dependency warnings.

Also separated api classes and testing helpers into
separate source sets in :common so that testing
classes may be restricted to test configurations.

common/src/main/java/google/registry/util/SystemClock.java

@@ -0,0 +1,37 @@
+// Copyright 2017 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.util;
+
+import static org.joda.time.DateTimeZone.UTC;
+
+import javax.annotation.concurrent.ThreadSafe;
+import javax.inject.Inject;
+import org.joda.time.DateTime;
+
+/** Clock implementation that proxies to the real system clock. */
+@ThreadSafe
+public class SystemClock implements Clock {
+
+  private static final long serialVersionUID = 5165372013848947515L;
+
+  @Inject
+  public SystemClock() {}
+
+  /** Returns the current time. */
+  @Override
+  public DateTime nowUtc() {
+    return DateTime.now(UTC);
+  }
+}