Add GN build targets for the rest of mojo/public

mojo/BUILD.gn

 # Copyright 2014 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.
 
 group("mojo") {
   deps = [
+    "//mojo/public/c/system",
+    "//mojo/public/cpp/application",
     "//mojo/public/cpp/bindings",
+    "//mojo/public/cpp/environment:standalone",
+    "//mojo/public/cpp/utility",
     "//mojo/public/interfaces/bindings/tests:test_interfaces",
     "//mojo/public/interfaces/service_provider:service_provider",
     "//mojo/public/js/bindings",
   ]
+
+  # TODO(cmasone): Android-only targets from mojo_public.gypi.
 }