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/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())