Revert "Revert "Unload oat files""

Fixed a race where two threads calling OatFile::Open could both use
dlopen on the host.

Bug: 22720414

This reverts commit 72da5e7461fec3b1e116050f2e6f233efb9c54f3.

Change-Id: I1636045b724944d2a09417527280784967957095
diff --git a/runtime/native/dalvik_system_DexFile.h b/runtime/native/dalvik_system_DexFile.h
index 7585ab9..77d219d 100644
--- a/runtime/native/dalvik_system_DexFile.h
+++ b/runtime/native/dalvik_system_DexFile.h
@@ -18,9 +18,13 @@
 #define ART_RUNTIME_NATIVE_DALVIK_SYSTEM_DEXFILE_H_
 
 #include <jni.h>
+#include <unistd.h>
 
 namespace art {
 
+constexpr size_t kOatFileIndex = 0;
+constexpr size_t kDexFileIndexStart = 1;
+
 class DexFile;
 
 void register_dalvik_system_DexFile(JNIEnv* env);