Allow to set the network used for an update attempt.

When triggering an update attempt in Android, we can force all the
traffic to go through a given network by passing a NETWORK_ID=<id>
in the extra key-value pair field.

Bug: 29921093
TEST=Called update_engine_client passing NETWORK_ID.

Change-Id: I13becdd133cccc912b542d48061fcfe9f566099a
diff --git a/network_selector_stub.cc b/network_selector_stub.cc
new file mode 100644
index 0000000..218d454
--- /dev/null
+++ b/network_selector_stub.cc
@@ -0,0 +1,44 @@
+//
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#include "update_engine/network_selector_stub.h"
+
+#include <base/logging.h>
+#include <brillo/make_unique_ptr.h>
+
+namespace chromeos_update_engine {
+
+namespace network {
+
+// Factory defined in network_selector.h.
+std::unique_ptr<NetworkSelectorInterface> CreateNetworkSelector() {
+  return brillo::make_unique_ptr(new NetworkSelectorStub());
+}
+
+}  // namespace network
+
+// Defined in network_selector_interface.h.
+const NetworkId kDefaultNetworkId = 0;
+
+bool NetworkSelectorStub::SetProcessNetwork(NetworkId network_id) {
+  if (network_id != kDefaultNetworkId) {
+    LOG(ERROR) << "SetProcessNetwork not implemented.";
+    return false;
+  }
+  return true;
+}
+
+}  // namespace chromeos_update_engine