Move YamlUtils to be under google.registry.util package

This makes it simpler to package google.registry.util as a separate project in
Gradle that can be depended upon by the proxy package. Currently the proxy
package depends on both google.registry.util and google.registry.config.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=221450085

java/google/registry/util/BUILD

@@ -21,5 +21,6 @@ java_library(
         "@com_ibm_icu_icu4j",
         "@javax_inject",
         "@joda_time",
+        "@org_yaml_snakeyaml",
     ],
 )