Pass -yaml to ds builds to generate _book.yaml version of reference TOC.

Change-Id: I1eced1670eb9d7892672b59f3b92833448b9949a
diff --git a/Android.mk b/Android.mk
index a03ab29..a016b84 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1054,6 +1054,7 @@
 		$(sample_groups) \
 		-useUpdatedTemplates \
 		-hdf android.hasSamples true \
+		-yaml _book.yaml \
 		-samplesdir $(samples_dir)
 
 LOCAL_DROIDDOC_CUSTOM_TEMPLATE_DIR:=build/tools/droiddoc/templates-sdk-dev