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/domain/DomainDeleteFlow.java

@@ -62,7 +62,6 @@ import google.registry.flows.custom.EntityChanges;
 import google.registry.model.ImmutableObject;
 import google.registry.model.billing.BillingEvent;
 import google.registry.model.domain.DomainBase;
-import google.registry.model.domain.DomainBase.Builder;
 import google.registry.model.domain.GracePeriod;
 import google.registry.model.domain.fee.BaseFee.FeeType;
 import google.registry.model.domain.fee.Credit;
@@ -149,7 +148,7 @@ public final class DomainDeleteFlow implements TransactionalFlow {
     flowCustomLogic.afterValidation(
         AfterValidationParameters.newBuilder().setExistingDomain(existingDomain).build());
     ImmutableSet.Builder<ImmutableObject> entitiesToSave = new ImmutableSet.Builder<>();
-    Builder builder;
+    DomainBase.Builder builder;
     if (existingDomain.getStatusValues().contains(StatusValue.PENDING_TRANSFER)) {
       builder =
           denyPendingTransfer(existingDomain, TransferStatus.SERVER_CANCELLED, now, clientId)

java/google/registry/flows/domain/DomainRestoreRequestFlow.java

@@ -46,7 +46,6 @@ import google.registry.flows.annotations.ReportingSpec;
 import google.registry.model.ImmutableObject;
 import google.registry.model.billing.BillingEvent;
 import google.registry.model.billing.BillingEvent.OneTime;
-import google.registry.model.billing.BillingEvent.OneTime.Builder;
 import google.registry.model.billing.BillingEvent.Reason;
 import google.registry.model.domain.DomainBase;
 import google.registry.model.domain.DomainCommand.Update;
@@ -258,7 +257,7 @@ public final class DomainRestoreRequestFlow implements TransactionalFlow  {
         .build();
   }
 
-  private Builder prepareBillingEvent(HistoryEntry historyEntry, Money cost, DateTime now) {
+  private OneTime.Builder prepareBillingEvent(HistoryEntry historyEntry, Money cost, DateTime now) {
     return new BillingEvent.OneTime.Builder()
         .setTargetId(targetId)
         .setClientId(clientId)

java/google/registry/flows/domain/DomainTransferApproveFlow.java

@@ -191,7 +191,7 @@ public final class DomainTransferApproveFlow implements TransactionalFlow {
             .setAutorenewPollMessage(Key.create(gainingClientAutorenewPollMessage))
             // Remove all the old grace periods and add a new one for the transfer.
             .setGracePeriods(
-                (billingEvent.isPresent())
+                billingEvent.isPresent()
                     ? ImmutableSet.of(
                         GracePeriod.forBillingEvent(GracePeriodStatus.TRANSFER, billingEvent.get()))
                     : ImmutableSet.of())