add api level 18 to javadoc 'since' tags

Change-Id: Iea2e13ef2fad47d6a19d380b5655aeeda0414930
diff --git a/Android.mk b/Android.mk
index 5bf4d41..8115472 100644
--- a/Android.mk
+++ b/Android.mk
@@ -442,6 +442,7 @@
     -since $(SRC_API_DIR)/15.txt 15 \
     -since $(SRC_API_DIR)/16.txt 16 \
     -since $(SRC_API_DIR)/17.txt 17 \
+    -since $(SRC_API_DIR)/18.txt 18 \
 		-werror -hide 113 \
 		-overview $(LOCAL_PATH)/core/java/overview.html