Use faster isAnnotationPresent check

Followup from https://github.com/square/moshi/pull/749#discussion_r235224202

moshi/src/main/java/com/squareup/moshi/ClassJsonAdapter.java

@@ -73,13 +73,18 @@ final class ClassJsonAdapter<T> extends JsonAdapter<T> {
       if (Modifier.isAbstract(rawType.getModifiers())) {
         throw new IllegalArgumentException("Cannot serialize abstract class " + rawType.getName());
       }
-      for (Annotation annotation : rawType.getDeclaredAnnotations()) {
-        if ("kotlin.Metadata".equals(annotation.annotationType().getName())) {
+      try {
+        //noinspection unchecked if the Class.forName works, the cast will work.
+        Class<? extends Annotation> metadataClass =
+            (Class<? extends Annotation>) Class.forName("kotlin.Metadata");
+        if (rawType.isAnnotationPresent(metadataClass)) {
           throw new IllegalArgumentException("Cannot serialize Kotlin type " + rawType.getName()
               + ". Reflective serialization of Kotlin classes without using kotlin-reflect has "
               + "undefined and unexpected behavior. Please use KotlinJsonAdapter from the "
               + "moshi-kotlin artifact or use code gen from the moshi-kotlin-codegen artifact.");
         }
+      } catch (ClassNotFoundException ignored) {
+
       }
 
       ClassFactory<Object> classFactory = ClassFactory.get(rawType);