Add support for enqueuing buffers in arbitrary order

Also added a very simple SharedBufferStack unit test.

Change-Id: I253dbbe98a53c966b78d22d4d6dd59f8aefc8c40
diff --git a/libs/surfaceflinger_client/tests/Android.mk b/libs/surfaceflinger_client/tests/Android.mk
new file mode 100644
index 0000000..5053e7d
--- /dev/null
+++ b/libs/surfaceflinger_client/tests/Android.mk
@@ -0,0 +1 @@
+include $(call all-subdir-makefiles)