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

javatests/google/registry/server/StaticResourceServlet.java

@@ -95,7 +95,7 @@ public final class StaticResourceServlet extends HttpServlet {
     String prefix = config.getInitParameter("prefix");
     verify(prefix.startsWith("/"));
     boolean isDirectory = Files.isDirectory(root);
-    verify(!isDirectory || isDirectory && prefix.endsWith("/"));
+    verify(!isDirectory || (isDirectory && prefix.endsWith("/")));
     fileServer = Optional.of(new FileServer(root, prefix));
   }