udpate refonly doc build to include new landing page for dev preview.

Change-Id: I088425ed1549dd3bae6a5a755884eb4c3cb0af4f
(cherry picked from commit aa6aa2929c546289c873b3d019de4616c2ddc620)
diff --git a/Android.mk b/Android.mk
index 9dd325a..242534b 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1073,7 +1073,7 @@
 
 static_doc_index_redirect := $(out_dir)/index.html
 $(static_doc_index_redirect): \
-	$(LOCAL_PATH)/docs/docs-documentation-redirect.html | $(ACP)
+	$(LOCAL_PATH)/docs/docs-preview-index.html | $(ACP)
 	$(hide) mkdir -p $(dir $@)
 	$(hide) $(ACP) $< $@