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/tldconfig/idn/LanguageValidator.java

@@ -15,8 +15,10 @@
 package google.registry.tldconfig.idn;
 
 import com.google.common.collect.ImmutableMap;
+import com.google.errorprone.annotations.Immutable;
 import java.util.Optional;
 
+@Immutable
 abstract class LanguageValidator {
 
   /** A registry of all known language validators keyed by their language code. */