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/cdm/src/wv_content_decryption_module.cpp

@@ -389,9 +389,9 @@ CdmEngine* WvContentDecryptionModule::EnsureCdmForIdentifier(
     // origin provided by the app and an identifier that uniquely identifies
     // this CDM. We concatenate all pieces of the CdmIdentifier in order to
     // create an ID that is unique to that identifier.
-    cdms_[identifier].file_system.SetOrigin(identifier.origin);
-    cdms_[identifier].file_system.SetIdentifier(identifier.spoid +
-                                                identifier.origin);
+    cdms_[identifier].file_system.set_origin(identifier.origin);
+    cdms_[identifier].file_system.set_identifier(identifier.spoid +
+                                                 identifier.origin);
 
     // Set the app package name for use by metrics.
     cdms_[identifier].cdm_engine->GetMetrics()->SetAppPackageName(