Set clock precision to milliseconds for Datetime->Instant migration (#2999)

Our existing precision is milliseconds so we want to stick with that for
Instants. If we want to increase the precision globally after that we can do so
all in one go post-migration, but for now, it would be a bad thing to have mixed
precision going on just depending on whether a class happens to be migrated yet
or not.

This PR also migrates all existing DateTime.nowUtc() calls to use the Clock
interface, so that when they are migrated they will get the benefit of this
precision-setting as well.

BUG= http://b/496985355

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

@@ -170,6 +170,12 @@ public abstract class DateTimeUtils {
     return (instant == null) ? null : new DateTime(instant.toEpochMilli(), DateTimeZone.UTC);
   }
 
+  /** Convert a java.time {@link java.time.Instant} to a joda {@link org.joda.time.Instant}. */
+  @Nullable
+  public static org.joda.time.Instant toJodaInstant(@Nullable java.time.Instant instant) {
+    return (instant == null) ? null : org.joda.time.Instant.ofEpochMilli(instant.toEpochMilli());
+  }
+
   public static Instant plusYears(Instant instant, int years) {
     return instant.atZone(ZoneOffset.UTC).plusYears(years).toInstant();
   }