Back button should be able to close apps

sky/services/intents/intents.mojom

 // Copyright 2015 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 intents;
 
 struct StringExtra {
   string name;
   string value;
 };
 
 struct ComponentName {
   string package_name;
   string class_name;
 };
 
 struct Intent {
   string action;
   string url;
   ComponentName? component;
   array<StringExtra>? string_extras;
 };
 
 // TODO(abarth): This interface seems very specific to Android. Do we want to
 //               have a higher-level abstraction here? Do we want a collection
 //               of services that only work on specific platforms? We need to
 //               figure out how to rationalize this interface across platforms.
 interface ActivityManager {
   startActivity(Intent intent);
+  finishCurrentActivity();
 };