Add the ability for Sky content to trigger Android intents

We can use these intents to move the demo launcher to the new world.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1187583002.

examples/raw/launcher.dart

+// 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.
+
+import "dart:math";
+import 'dart:sky';
+
+import 'package:sky/framework/shell.dart' as shell;
+import 'package:mojom/intents/intents.mojom.dart';
+
+Picture draw(int a, int r, int g, int b) {
+  double width = view.width;
+  double height = view.height;
+
+  PictureRecorder recorder = new PictureRecorder(width, height);
+  double radius = min(width, height) * 0.45;
+
+  Paint paint = new Paint()..color = new Color.fromARGB(a, r, g, b);
+  recorder.drawRect(new Rect.fromSize(new Size(width, height)), paint);
+  return recorder.endRecording();
+}
+
+bool handleEvent(Event event) {
+  if (event.type == "pointerdown") {
+    view.picture = draw(255, 0, 0, 255);
+    view.scheduleFrame();
+    return true;
+  }
+
+  if (event.type == "pointerup") {
+    view.picture = draw(255, 255, 255, 0);
+    view.scheduleFrame();
+
+    ActivityManagerProxy activityManager = new ActivityManagerProxy.unbound();
+    Intent intent = new Intent()
+      ..action = 'android.intent.action.VIEW'
+      ..url = 'sky://localhost:9888/sky/examples/raw/hello_world.dart';
+    shell.requestService(null, activityManager);
+    activityManager.ptr.startActivity(intent);
+    return true;
+  }
+
+  if (event.type == "back") {
+    print("Pressed back button.");
+    return true;
+  }
+
+  return false;
+}
+
+void main() {
+  print("Hello, world");
+  view.picture = draw(255, 255, 255, 0);
+  view.scheduleFrame();
+
+  view.setEventCallback(handleEvent);
+}