Refactor file_store to use smart pointers

Bug: b/119276649
Merge from: http://go/wvgerrit/66367
Test: Android, CE CDM, 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/oemcrypto/ref/src/oemcrypto_engine_ref.cpp

@@ -118,7 +118,7 @@ time_t CryptoEngine::RollbackCorrectedOfflineTime() {
   // Use the device key for encrypt/decrypt.
   const std::vector<uint8_t>& key = DeviceRootKey();
 
-  wvcdm::File* file;
+  std::unique_ptr<wvcdm::File> file;
   std::string path;
   // Note: this path is OK for a real implementation, but using security level 1
   // would be better.
@@ -140,7 +140,6 @@ time_t CryptoEngine::RollbackCorrectedOfflineTime() {
       return time(NULL);
     }
     file->Read(reinterpret_cast<char*>(&encrypted_buffer[0]), sizeof(TimeInfo));
-    file->Close();
     // Decrypt the encrypted TimeInfo buffer.
     AES_KEY aes_key;
     AES_set_decrypt_key(&key[0], 128, &aes_key);
@@ -180,7 +179,6 @@ time_t CryptoEngine::RollbackCorrectedOfflineTime() {
     return time(NULL);
   }
   file->Write(reinterpret_cast<char*>(&encrypted_buffer[0]), sizeof(TimeInfo));
-  file->Close();
 
   // Return time with offset.
   return current_time;