Use javax.lang.model APIs instead of javac implementation classes (#577)

Summary:
`com.sun.tools.javac.code.Type.ClassType` is an internal javac API that should not be used in annotation processors, `javax.lang.model.type.DeclaredType` is the corresponding public interface.
Pull Request resolved: #577

Test Plan: Existing tests.

Reviewed By: colriot

Differential Revision: D16666070

Pulled By: muraziz

fbshipit-source-id: a38b80d5b7a9b380d8100cb9ca4655ddebd61234

litho-processor/src/main/java/com/facebook/litho/specmodels/processor/TestTargetExtractor.java

@@ -17,9 +17,9 @@
 package com.facebook.litho.specmodels.processor;
 
 import com.facebook.litho.annotations.TestSpec;
-import com.sun.tools.javac.code.Type;
 import javax.annotation.Nullable;
 import javax.lang.model.element.TypeElement;
+import javax.lang.model.type.DeclaredType;
 import javax.lang.model.type.MirroredTypeException;
 import javax.lang.model.type.TypeMirror;
 
@@ -31,7 +31,7 @@ public static TypeElement getTestSpecValue(TypeElement element) {
       element.getAnnotation(TestSpec.class).value();
     } catch (MirroredTypeException e) {
       final TypeMirror typeMirror = e.getTypeMirror();
-      return (TypeElement) ((Type.ClassType) typeMirror).asElement();
+      return (TypeElement) ((DeclaredType) typeMirror).asElement();
     }
 
     return null;