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/oemcrypto/mock/src/oemcrypto_old_usage_table_mock.cpp

@@ -59,7 +59,7 @@ OldUsageTable::OldUsageTable(CryptoEngine *ce) {
 
   // Load saved table.
   wvcdm::FileSystem* file_system = ce->file_system();
-  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.
@@ -90,7 +90,6 @@ OldUsageTable::OldUsageTable(CryptoEngine *ce) {
     return;
   }
   file->Read(reinterpret_cast<char *>(&encrypted_buffer[0]), file_size);
-  file->Close();
 
   // Verify the signature of the usage table file.
 
@@ -141,7 +140,6 @@ OldUsageTable::OldUsageTable(CryptoEngine *ce) {
     return;
   }
   file->Read(reinterpret_cast<char *>(&generation_), sizeof(int64_t));
-  file->Close();
   if (stored_table->generation == generation_ + 1) {
     if (LogCategoryEnabled(kLoggingTraceUsageTable)) {
       LOGW("OldUsageTable: File is one generation old.  Acceptable rollback.");