add <meta> tag for "body_class" that carries the api level
This is necessary because DevSite strips out the class names
we normally apply in the <body> tag. So now DevSite will read
this meta tag and apply these class names to the <body> tag
as it builds the page.
Bug: 36226534

Change-Id: I225718f3c1b11614b45a8db510ac92bcf2f56d30
diff --git a/res/assets/templates-sdk/head_tag.cs b/res/assets/templates-sdk/head_tag.cs
index 667b8b2..6dbe393 100644
--- a/res/assets/templates-sdk/head_tag.cs
+++ b/res/assets/templates-sdk/head_tag.cs
@@ -170,7 +170,9 @@
       if:excludeFromSuggestions ?>
   <meta name="hide_from_search_suggest" value="true" /><?cs
       /if ?><?cs
-
+      if:reference ?>
+  <meta name="body_class" value="api apilevel-<?cs var:class.since ?><?cs var:package.since ?>" /><?cs
+      /if ?><?cs
       if:!dac ?>
   <meta name="hide_page_heading" value="true" /><?cs
       /if ?><?cs