Prefer re2 over java.util.regex

Java's stock regex implementation doesn't guarantee linear time
complexity which makes it a security liability.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=121159875

java/google/registry/ui/forms/FormFields.java

@@ -21,8 +21,7 @@ import static java.util.Locale.getISOCountries;
 
 import com.google.common.base.Function;
 import com.google.common.collect.ImmutableSet;
-
-import java.util.regex.Pattern;
+import com.google.re2j.Pattern;
 
 import javax.annotation.Nullable;