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"

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

java/google/registry/flows/contact/ContactUpdateFlow.java

@@ -39,7 +39,6 @@ import google.registry.flows.exceptions.ResourceHasClientUpdateProhibitedExcepti
 import google.registry.model.contact.ContactCommand.Update;
 import google.registry.model.contact.ContactCommand.Update.Change;
 import google.registry.model.contact.ContactResource;
-import google.registry.model.contact.ContactResource.Builder;
 import google.registry.model.contact.PostalInfo;
 import google.registry.model.domain.metadata.MetadataExtension;
 import google.registry.model.eppcommon.AuthInfo;
@@ -109,7 +108,7 @@ public final class ContactUpdateFlow implements TransactionalFlow {
         .setXmlBytes(null)  // We don't want to store contact details in the history entry.
         .setParent(Key.create(existingContact));
     checkSameValuesNotAddedAndRemoved(statusesToAdd, statusToRemove);
-    Builder builder = existingContact.asBuilder();
+    ContactResource.Builder builder = existingContact.asBuilder();
     Change change = command.getInnerChange();
     // The spec requires the following behaviors:
     //   * If you update part of a postal info, the fields that you didn't update are unchanged.