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/src/file_store.cpp

@@ -171,6 +171,11 @@ ssize_t FileSystem::FileSize(const std::string& in_path) {
     return -1;
 }
 
+bool FileSystem::List(const std::string& path,
+                      std::vector<std::string>* filenames) {
+  return FileUtils::List(GetFileNameForIdentifier(path, origin_), filenames);
+}
+
 void FileSystem::SetOrigin(const std::string& origin) { origin_ = origin; }
 
 void FileSystem::SetIdentifier(const std::string& identifier) {