GN: Add mojo_shell_tests, get more things working on android

mojo/system.gni

Index: mojo/system.gni
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+# 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.
+
+# The following mojo_system-prefixed variables are used to express a dependency
+# on the mojo system APIs.
+#
+# In an is_component_build build, everything can link against
+# mojo_system_impl because it is built as a shared library. However, in a static
+# build, mojo_system_impl is linked into an executable (e.g.,
+# mojo_shell), and must be injected into other shared libraries (i.e., Mojo
+# Apps) that need the mojo system API.
+#
+# For component targets, add mojo_system_for_component to your deps section.
+# For shared_library targets (e.g., a Mojo App), add
+# mojo_system_for_shared_library to your deps
+
+if (is_component_build) {
+  mojo_system_for_component = [ "//mojo/system" ]
+  mojo_system_for_shared_library = [ "//mojo/system" ]
+} else {
+  mojo_system_for_component = [] # nothing to link against here
+  mojo_system_for_shared_library = [ "//mojo/public/c/system" ]
+}