add api 20

DO NOT MERGE

Change-Id: Iae685158ac9c08e3a074e36d60004faf995fd1c7
diff --git a/Android.mk b/Android.mk
index 8fdcea2..a8e072c 100644
--- a/Android.mk
+++ b/Android.mk
@@ -535,6 +535,7 @@
     -since $(SRC_API_DIR)/17.txt 17 \
     -since $(SRC_API_DIR)/18.txt 18 \
     -since $(SRC_API_DIR)/19.txt 19 \
+    -since $(SRC_API_DIR)/20.txt 20 \
 		-werror -hide 113 \
 		-overview $(LOCAL_PATH)/core/java/overview.html