[Device Service] Move Gamepad Blink headers to be part of the Gamepad client library

device/gamepad/public/cpp/gamepad.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "device/gamepad/public/cpp/gamepad.h"
+
+namespace device {
+
+Gamepad::Gamepad()
+    : connected(false),
+      timestamp(0),
+      axes_length(0),
+      buttons_length(0),
+      display_id(0) {
+  id[0] = 0;
+  mapping[0] = 0;
+}
+
+Gamepad::Gamepad(const Gamepad& other) = default;
+
+}  // namespace device