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/oat_file.cc b/runtime/oat_file.cc
index c666441..446a004 100644
--- a/runtime/oat_file.cc
+++ b/runtime/oat_file.cc
@@ -43,6 +43,7 @@
 #include "base/stl_util.h"
 #include "base/systrace.h"
 #include "base/unix_file/fd_file.h"
+#include "dex/art_dex_file_loader.h"
 #include "dex/dex_file_loader.h"
 #include "dex/dex_file_types.h"
 #include "dex/standard_dex_file.h"
@@ -1666,14 +1667,15 @@
   ScopedTrace trace(__PRETTY_FUNCTION__);
   static constexpr bool kVerify = false;
   static constexpr bool kVerifyChecksum = false;
-  return DexFileLoader::Open(dex_file_pointer_,
-                             FileSize(),
-                             dex_file_location_,
-                             dex_file_location_checksum_,
-                             this,
-                             kVerify,
-                             kVerifyChecksum,
-                             error_msg);
+  const ArtDexFileLoader dex_file_loader;
+  return dex_file_loader.Open(dex_file_pointer_,
+                              FileSize(),
+                              dex_file_location_,
+                              dex_file_location_checksum_,
+                              this,
+                              kVerify,
+                              kVerifyChecksum,
+                              error_msg);
 }
 
 uint32_t OatFile::OatDexFile::GetOatClassOffset(uint16_t class_def_index) const {