Clean up some code quality issues

This removes some qualifiers that aren't necessary (e.g. public/abstract on interfaces, private on enum constructors, final on private methods, static on nested interfaces/enums), uses Java 8 lambdas and features where that's an improvement

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=177182945

javatests/google/registry/model/translators/StatusValueAdapterTest.java

@@ -65,7 +65,7 @@ public class StatusValueAdapterTest {
                 .build()),
             ValidationMode.LENIENT),
         UTF_8);
-    assertThat(marshalled.toString()).contains("<host:status s=\"clientUpdateProhibited\"/>");
+    assertThat(marshalled).contains("<host:status s=\"clientUpdateProhibited\"/>");
   }
 
   private StatusValue unmarshal(String statusValueXml) throws Exception {