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

javatests/google/registry/testing/mapreduce/MapreduceTestCase.java

@@ -30,6 +30,7 @@ import com.google.appengine.tools.pipeline.impl.servlets.PipelineServlet;
 import com.google.appengine.tools.pipeline.impl.servlets.TaskHandler;
 import com.google.apphosting.api.ApiProxy;
 import com.google.common.base.CharMatcher;
+import com.google.common.base.Splitter;
 import com.google.common.flogger.FluentLogger;
 import google.registry.mapreduce.MapreduceRunner;
 import google.registry.testing.AppEngineRule;
@@ -224,11 +225,11 @@ public abstract class MapreduceTestCase<T> extends ShardableTestCase {
       throws UnsupportedEncodingException {
     Map<String, String> result = new HashMap<>();
 
-    String[] params = requestBody.split("&");
+    Iterable<String> params = Splitter.on('&').split(requestBody);
     for (String param : params) {
-      String[] pair = param.split("=");
-      String name = pair[0];
-      String value = URLDecoder.decode(pair[1], "UTF-8");
+      List<String> pair = Splitter.on('=').splitToList(param);
+      String name = pair.get(0);
+      String value = URLDecoder.decode(pair.get(1), "UTF-8");
       if (result.containsKey(name)) {
         throw new IllegalArgumentException("Duplicate parameter: " + requestBody);
       }