docs: update makefile to show javadocs diffs for api level 12

Change-Id: I1ac584db778ec14b18129785126159f692459158
diff --git a/Android.mk b/Android.mk
index a28bde2..b266061 100644
--- a/Android.mk
+++ b/Android.mk
@@ -371,6 +371,7 @@
     -since ./frameworks/base/api/9.xml 9 \
     -since ./frameworks/base/api/10.xml 10 \
     -since ./frameworks/base/api/11.xml 11 \
+    -since ./frameworks/base/api/12.xml 12 \
 		-werror -hide 113 \
 		-overview $(LOCAL_PATH)/core/java/overview.html