Add gn support for generating mojo_shell_apk

mojo/services/native_viewport/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.
 
 import("//build/config/ui.gni")
 
-if (!is_android) {
+if (is_android) {
+  import("//build/config/android/config.gni")
+  import("//build/config/android/rules.gni")
+
+  android_library("native_viewport_java") {
+    deps = [
+      "//base:base_java",
+    ]
+    java_files = [
+      "android/src/org/chromium/mojo/PlatformViewportAndroid.java",
+    ]
+  }
+} else {
   shared_library("native_viewport") {
     output_name = "native_viewport_service"
 
     deps = [
       ":lib",
       "//base",
       "//mojo/application",
       "//mojo/public/c/system:for_shared_library",
       "//mojo/public/cpp/bindings:bindings",
       "//mojo/services/public/interfaces/native_viewport",
@@ skipping 58 matching lines @@
     deps += [
       "//ui/events/platform/x11",
       "//ui/platform_window/x11",
     ]
   }
 
   if (use_ozone) {
     sources += [ "platform_viewport_ozone.cc" ]
   }
 }