Refactor file_store to use smart pointers

Bug: b/119276649
Merge of http://go/wvgerrit/68903
Test: Android + Linux unit tests

The FileSystem interface as it exists expects an Open for a file and
then a Close when finished. However, the Close doesn't delete the file
itself and depending on the platform, the underlying impl_ as well,
leading to a memory leak. To fix this leak as well as harden against
future memory issues, this change refactors the interface to shift away
from raw pointers and towards smart pointers.

Change-Id: I7a7132ea95cd3775796a540f510b698f4f27dd24

libwvdrmengine/level3/include/level3_file_system_android.h

@@ -13,6 +13,8 @@
 
 #include "level3_file_system.h"
 
+#include <memory>
+
 #include "file_store.h"
 
 namespace wvoec3 {
@@ -29,7 +31,7 @@ class OEMCrypto_Level3AndroidFileSystem : public OEMCrypto_Level3FileSystem {
 
  private:
   std::string base_path_;
-  wvcdm::FileSystem *file_system_;
+  std::unique_ptr<wvcdm::FileSystem> file_system_;
 };
 
 }  // namespace wvoec3