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/xml/DateAdapterTest.java

@@ -51,14 +51,14 @@ public class DateAdapterTest {
 
   @Test
   public void testUnmarshalEmpty() {
-    assertThat((new DateAdapter()).unmarshal(null)).isNull();
-    assertThat((new DateAdapter()).unmarshal("")).isNull();
+    assertThat(new DateAdapter().unmarshal(null)).isNull();
+    assertThat(new DateAdapter().unmarshal("")).isNull();
   }
 
   @Test
   public void testUnmarshalInvalid() {
     assertThrows(
         IllegalArgumentException.class,
-        () -> assertThat((new DateAdapter()).unmarshal("oh my goth")).isNull());
+        () -> assertThat(new DateAdapter().unmarshal("oh my goth")).isNull());
   }
 }