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/xml/BUILD

@@ -20,5 +20,6 @@ java_library(
         "//java/com/google/common/io",
         "//third_party/java/joda_time",
         "//third_party/java/jsr305_annotations",
+        "//third_party/java/re2j",
     ],
 )

java/google/registry/xml/XmlFragmentMarshaller.java

@@ -18,8 +18,9 @@ import static com.google.common.base.Throwables.propagateIfInstanceOf;
 import static com.google.common.base.Verify.verify;
 import static java.nio.charset.StandardCharsets.UTF_8;
 
+import com.google.re2j.Pattern;
+
 import java.io.ByteArrayOutputStream;
-import java.util.regex.Pattern;
 
 import javax.annotation.concurrent.NotThreadSafe;
 import javax.xml.bind.JAXBContext;