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/runtime.h b/runtime/runtime.h
index abccb44..fd486f9 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -576,7 +576,7 @@
 
   OatFileManager& GetOatFileManager() const {
     DCHECK(oat_file_manager_ != nullptr);
-    return *oat_file_manager_.get();
+    return *oat_file_manager_;
   }
 
  private:
@@ -777,7 +777,7 @@
   std::string fingerprint_;
 
   // Oat file manager, keeps track of what oat files are open.
-  std::unique_ptr<OatFileManager> oat_file_manager_;
+  OatFileManager* oat_file_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(Runtime);
 };