add -since flag for api level 19 to filter javadocs

Change-Id: I96e7e31f93b1e3c3877a342b84fb069bda5d92e3
diff --git a/Android.mk b/Android.mk
index 49dc6af..8bc4d5d 100644
--- a/Android.mk
+++ b/Android.mk
@@ -523,6 +523,7 @@
     -since $(SRC_API_DIR)/16.txt 16 \
     -since $(SRC_API_DIR)/17.txt 17 \
     -since $(SRC_API_DIR)/18.txt 18 \
+    -since $(SRC_API_DIR)/19.txt 19 \
 		-werror -hide 113 \
 		-overview $(LOCAL_PATH)/core/java/overview.html