[DeviceService] Move PowerMonitor interfaces, client lib, impl to live in Device Service

device/power_monitor/public/interfaces/power_monitor.mojom

-// Copyright 2016 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.
-
-module device.mojom;
-
-interface PowerMonitor {
-  // Set client that will be notified on PowerStateChange, Suspend and Resume.
-  SetClient(PowerMonitorClient client);
-};
-
-interface PowerMonitorClient {
-  // Notification of a change in power status of the computer, such
-  // as from switching between battery and A/C power.
-  PowerStateChange(bool on_battery_power);
-
-  // Notification that the system is suspending.
-  Suspend();
-
-  // Notification that the system is resuming.
-  Resume();
-};