Make AddressInfo.builder return Builder instead of BuilderImpl

Author: mziccard

gcloud-java-compute/src/main/java/com/google/gcloud/compute/AddressInfo.java

@@ -531,7 +531,7 @@ Address toPb() {
   /**
    * Returns a builder for the {@code AddressInfo} object given it's identity.
    */
-  public static BuilderImpl builder(AddressId addressId) {
+  public static Builder builder(AddressId addressId) {
     return new BuilderImpl().addressId(addressId);
   }