bluetooth: Standardize Bluetooth adapter access in Adapter service.

device/bluetooth/public/interfaces/adapter.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 bluetooth.mojom;
 
 struct DeviceInfo {
   string? name;
   string name_for_display;
   string id;
@@ skipping 10 matching lines @@
   SetClient(AdapterClient client);
 };
 
 interface AdapterClient {
   // Called the first time a device is discovered.
   DeviceAdded(DeviceInfo device);
 
   // Called after the device hasn't been seen for 3 minutes.
   DeviceRemoved(DeviceInfo device);
 };
+
+interface AdapterFactory {
+  // Gets an Adapter interface. Returns null if Bluetooth is not supported.
+  GetAdapter() => (Adapter? adapter);
+};