update L Preview message and disallow crawling /preview

Change-Id: I08a81a3057874a0c19f863b0d0c3e9dafa757e6d
diff --git a/docs/html/robots.txt b/docs/html/robots.txt
index f522220..ab379bb 100644
--- a/docs/html/robots.txt
+++ b/docs/html/robots.txt
@@ -15,4 +15,5 @@
 Disallow: /guide/tutorials/
 Disallow: /guide/samples/
 Disallow: /community/
+Disallow: /preview/
 Sitemap: http://developer.android.com/sitemap.txt