Add gn support for generating mojo_shell_apk

mojo/services/native_viewport/BUILD.gn

Index: mojo/services/native_viewport/BUILD.gn
diff --git a/mojo/services/native_viewport/BUILD.gn b/mojo/services/native_viewport/BUILD.gn
index ed428c459535aa829aea0d1ffcf98b1d02dd417c..43b685f5a709a714c14e3d6b9bd24417a0727183 100644
--- a/mojo/services/native_viewport/BUILD.gn
+++ b/mojo/services/native_viewport/BUILD.gn
@@ -4,7 +4,19 @@
 
 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"