Make our Clock util Serializable

It doesn't entirely make semantic sense, since the actual state of the
SystemClock isn't being preserved, but it makes injection into serializable
classes (e.g. mapreduces) much simpler, so it's worth doing.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=201755949

javatests/google/registry/testing/FakeClock.java

@@ -19,7 +19,6 @@ import static org.joda.time.DateTimeZone.UTC;
 import static org.joda.time.Duration.millis;
 
 import google.registry.util.Clock;
-import java.io.Serializable;
 import java.util.concurrent.atomic.AtomicLong;
 import javax.annotation.concurrent.ThreadSafe;
 import org.joda.time.DateTime;
@@ -28,7 +27,7 @@ import org.joda.time.ReadableInstant;
 
 /** A mock clock for testing purposes that supports telling, setting, and advancing the time. */
 @ThreadSafe
-public final class FakeClock implements Clock, Serializable {
+public final class FakeClock implements Clock {
 
   private static final long serialVersionUID = 675054721685304599L;