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/model/domain/GracePeriod.java

@@ -100,7 +100,8 @@ public class GracePeriod extends ImmutableObject {
        @Nullable Key<BillingEvent.Recurring> billingEventRecurring) {
     checkArgument((billingEventOneTime == null) || (billingEventRecurring == null),
         "A grace period can have at most one billing event");
-    checkArgument((billingEventRecurring != null) == (GracePeriodStatus.AUTO_RENEW.equals(type)),
+    checkArgument(
+        (billingEventRecurring != null) == GracePeriodStatus.AUTO_RENEW.equals(type),
         "Recurring billing events must be present on (and only on) autorenew grace periods");
     GracePeriod instance = new GracePeriod();
     instance.type = checkArgumentNotNull(type);