examples/provider/wifi_manager: find iface name

- remove hardcoded wlan0
- use ForkCmd instead of std::system

Change-Id: If32310e76aa3c7d9a6482f60fdf2c4f48471a021
Reviewed-on: https://weave-review.googlesource.com/1888
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
diff --git a/examples/provider/wifi_manager.h b/examples/provider/wifi_manager.h
index c043523..2bfc5ca 100644
--- a/examples/provider/wifi_manager.h
+++ b/examples/provider/wifi_manager.h
@@ -48,6 +48,7 @@
   provider::TaskRunner* task_runner_{nullptr};
   EventNetworkImpl* network_{nullptr};
   base::WeakPtrFactory<WifiImpl> weak_ptr_factory_{this};
+  std::string iface_;
 };
 
 }  // namespace examples