Create dex subdirectory

Move all the DexFile related source to a common subdirectory dex/ of
runtime.

Bug: 71361973
Test: make -j 50 test-art-host
Change-Id: I59e984ed660b93e0776556308be3d653722f5223
diff --git a/runtime/string_reference.h b/runtime/string_reference.h
index 24a4253..97661c6 100644
--- a/runtime/string_reference.h
+++ b/runtime/string_reference.h
@@ -21,9 +21,9 @@
 
 #include <android-base/logging.h>
 
-#include "dex_file-inl.h"
-#include "dex_file_reference.h"
-#include "dex_file_types.h"
+#include "dex/dex_file-inl.h"
+#include "dex/dex_file_reference.h"
+#include "dex/dex_file_types.h"
 #include "utf-inl.h"
 
 namespace art {