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/oat.h b/runtime/oat.h
index 7d1bf85..6d4f18b 100644
--- a/runtime/oat.h
+++ b/runtime/oat.h
@@ -22,7 +22,7 @@
 #include "arch/instruction_set.h"
 #include "base/macros.h"
 #include "compiler_filter.h"
-#include "dex_file.h"
+#include "dex/dex_file.h"
 #include "safe_map.h"
 
 namespace art {