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/common_compiler_test.cc b/compiler/common_compiler_test.cc
index 96d90bb..8ffc86e 100644
--- a/compiler/common_compiler_test.cc
+++ b/compiler/common_compiler_test.cc
@@ -165,7 +165,7 @@
                                               method_inliner_map_.get(),
                                               compiler_kind, instruction_set,
                                               instruction_set_features_.get(),
-                                              true, new std::set<std::string>, nullptr,
+                                              true, new std::unordered_set<std::string>, nullptr,
                                               2, true, true, "", timer_.get(), -1, ""));
   }
   // We typically don't generate an image in unit tests, disable this optimization by default.