Implement Cdm::listStoredLicenses()

[ Merge of http://go/wvgerrit/23600 ]

This adds a new entry to IStorage:: -

  bool list(std::vector<std::string> file_names)

It returns the name of each file in the (origin-specific) file system.

b/34628115

Uses the current file system (origin-specific) bound to the CDM. Returns
the list of stored licenses (key_set_ids) in vector output parameter.

Test: verified by unittests on angler.

Change-Id: I988556b27c2a4b75f52b59bcd78cfeaddd649acd

libwvdrmengine/cdm/core/include/cdm_session.h

@@ -5,6 +5,7 @@
 
 #include <set>
 #include <string>
+#include <vector>
 
 #include "crypto_session.h"
 #include "device_files.h"
@@ -119,6 +120,7 @@ class CdmSession {
   // release the underlying crypto session) rather than call this method.
   virtual CdmResponseType ReleaseCrypto();
 
+  // Delete current license and matching usage record
   bool DeleteLicense();
 
   // Generate unique ID for each new session.