GN: Add mojo_shell_tests, get more things working on android

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/common",
     "//mojo/public",
     "//mojo/services",
     "//mojo/shell:mojo_shell",
+    "//mojo/shell:mojo_shell_tests",
   ]
 }
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 
   generate_jni("jni_headers") {
     sources = [
       "android/javatests/src/org/chromium/mojo/MojoTestCase.java",
       "android/system/src/org/chromium/mojo/system/impl/CoreImpl.java",
       "services/native_viewport/android/src/org/chromium/mojo/NativeViewportAndroid.java",
     ]
 
     jni_package = "mojo"
   }
 }