libmojo: Add test_embedder.cc

This adds `mojo::edk::test::Shutdown()`.

Bug: 38506537
Test: m
Exempt-From-Owner-Approval: Obtained +2 from original reviewer
Change-Id: Iad6885cc19e7b52f5a60f9a8a1b99f3d02e868b8
(cherry picked from commit d29c46117fd941e00db64a10172b9589abb6b73c)
diff --git a/Android.mk b/Android.mk
index c54e37e..e25dbc6 100644
--- a/Android.mk
+++ b/Android.mk
@@ -87,6 +87,7 @@
 	mojo/edk/embedder/platform_handle.cc \
 	mojo/edk/embedder/platform_handle_utils_posix.cc \
 	mojo/edk/embedder/platform_shared_buffer.cc \
+	mojo/edk/embedder/test_embedder.cc \
 	mojo/edk/system/awakable_list.cc \
 	mojo/edk/system/broker_host_posix.cc \
 	mojo/edk/system/broker_posix.cc \