Add ToString() to Modules

Add ToString() to Module and classes inherit Module to help debug.

Bug: 144304945

Test: cert
Change-Id: I8535c6ddb182fd26a7c62b30679cfb90a5168ebe
diff --git a/gd/security/security_module.cc b/gd/security/security_module.cc
index e1d01d7..58b1f8c 100644
--- a/gd/security/security_module.cc
+++ b/gd/security/security_module.cc
@@ -62,6 +62,10 @@
   pimpl_.reset();
 }
 
+std::string SecurityModule::ToString() const {
+  return "Security Module";
+}
+
 std::unique_ptr<SecurityManager> SecurityModule::GetSecurityManager() {
   return std::unique_ptr<SecurityManager>(
       new SecurityManager(pimpl_->security_handler_, &pimpl_->security_manager_impl));