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/util/DiffUtils.java

@@ -138,7 +138,7 @@ public final class DiffUtils {
           && ((DiffPair) value).b instanceof Set) {
         DiffPair pair = ((DiffPair) value);
         String prettyLineDiff = prettyPrintSetDiff((Set<?>) pair.a, (Set<?>) pair.b) + "\n";
-        output = newPath + ((prettyLineDiff.startsWith("\n")) ? ":" : ": ") + prettyLineDiff;
+        output = newPath + (prettyLineDiff.startsWith("\n") ? ":" : ": ") + prettyLineDiff;
       } else {
         output = newPath + ": " + value + "\n";
       }