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

services/device/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();
 };