Create an ART-independent DexFileLoader

Opening DEX files should not rely on instantiating a runtime or having a
large number of dependencies on runtime components.  This CL makes
DexFileLoader a stub class that is independent of ART, and introduces a
subclass ArtDexFileLoader that contains the current implementations.

Bug: 22322814
Test: make -j 50 test-art-host
Change-Id: Ia6e92ae93c347057ea0c10455525239cbbe42c03
diff --git a/runtime/dex/code_item_accessors_test.cc b/runtime/dex/code_item_accessors_test.cc
index b29d10b..2e21956 100644
--- a/runtime/dex/code_item_accessors_test.cc
+++ b/runtime/dex/code_item_accessors_test.cc
@@ -19,6 +19,7 @@
 #include <memory>
 
 #include "common_runtime_test.h"
+#include "art_dex_file_loader.h"
 #include "dex_file_loader.h"
 #include "mem_map.h"
 
@@ -44,13 +45,13 @@
     StandardDexFile::WriteMagic(map->Begin());
     StandardDexFile::WriteCurrentVersion(map->Begin());
   }
-  std::unique_ptr<const DexFile> dex(
-      DexFileLoader::Open("location",
-                          /*location_checksum*/ 123,
-                          std::move(map),
-                          /*verify*/false,
-                          /*verify_checksum*/false,
-                          &error_msg));
+  const ArtDexFileLoader dex_file_loader;
+  std::unique_ptr<const DexFile> dex(dex_file_loader.Open("location",
+                                                          /*location_checksum*/ 123,
+                                                          std::move(map),
+                                                          /*verify*/false,
+                                                          /*verify_checksum*/false,
+                                                          &error_msg));
   CHECK(dex != nullptr) << error_msg;
   return dex;
 }