ART: Change image_classes and compiled_classes to unordered set

These lists can be large, and the soon-to-follow compiled_methods
will be potentially even larger. Use a hash set instead of a tree
set.

Change-Id: I7d25c075540f47771354c6f49928b4fd0c76eb2e
diff --git a/compiler/image_writer.cc b/compiler/image_writer.cc
index 670c897..a99ef34 100644
--- a/compiler/image_writer.cc
+++ b/compiler/image_writer.cc
@@ -776,7 +776,7 @@
 }
 
 void ImageWriter::DumpImageClasses() {
-  const std::set<std::string>* image_classes = compiler_driver_.GetImageClasses();
+  auto image_classes = compiler_driver_.GetImageClasses();
   CHECK(image_classes != NULL);
   for (const std::string& image_class : *image_classes) {
     LOG(INFO) << " " << image_class;