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/flows/host/HostCreateFlow.java

@@ -45,7 +45,6 @@ import google.registry.model.eppoutput.CreateData.HostCreateData;
 import google.registry.model.eppoutput.EppResponse;
 import google.registry.model.host.HostCommand.Create;
 import google.registry.model.host.HostResource;
-import google.registry.model.host.HostResource.Builder;
 import google.registry.model.index.EppResourceIndex;
 import google.registry.model.index.ForeignKeyIndex;
 import google.registry.model.ofy.ObjectifyService;
@@ -119,7 +118,7 @@ public final class HostCreateFlow implements TransactionalFlow {
           : new UnexpectedExternalHostIpException();
     }
     HostResource newHost =
-        new Builder()
+        new HostResource.Builder()
             .setCreationClientId(clientId)
             .setPersistedCurrentSponsorClientId(clientId)
             .setFullyQualifiedHostName(targetId)