Fix various Error Prone errors that were found by the FOSS build

Most common:
- Unnecessary parentheses and operator precedence clarify (self-explanatory)
- Reference equality--there were a few instances of using == or != improperly
- Qualification of Builder (and similar) imports so that it's clear which type of Builder we're referring to
- Marking some immutable classes with @Immutable since EP desires that all enums be deeply immutable
- String.split() having "surprising behavior"

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=230971531

java/google/registry/tools/javascrap/PopulateNullRegistrarFieldsCommand.java

@@ -21,7 +21,6 @@ import static google.registry.model.ofy.ObjectifyService.ofy;
 import com.beust.jcommander.Parameters;
 import com.google.common.collect.ImmutableList;
 import google.registry.model.registrar.Registrar;
-import google.registry.model.registrar.Registrar.Builder;
 import google.registry.model.registrar.RegistrarAddress;
 import google.registry.tools.MutatingCommand;
 import java.util.Objects;
@@ -40,7 +39,7 @@ public class PopulateNullRegistrarFieldsCommand extends MutatingCommand {
   @Override
   protected void init() {
     for (Registrar registrar : ofy().load().type(Registrar.class).ancestor(getCrossTldKey())) {
-      Builder changeBuilder = registrar.asBuilder();
+      Registrar.Builder changeBuilder = registrar.asBuilder();
       changeBuilder.setRegistrarName(
           firstNonNull(registrar.getRegistrarName(), registrar.getClientId()));