docs: Fix reference docs build to indicate N changes

Change-Id: I4cd64a0452e97aa87a86fffd1c9f9ba3d99eb5c3
(cherry picked from commit f6c304579d673135e0df23f887aa068fb91db0f3)

Android.mk

@@ -827,6 +827,7 @@ framework_docs_LOCAL_DROIDDOC_OPTIONS := \
     -since $(SRC_API_DIR)/21.txt 21 \
     -since $(SRC_API_DIR)/22.txt 22 \
     -since $(SRC_API_DIR)/23.txt 23 \
+    -since ./frameworks/base/api/current.txt N \
 		-werror -hide 111 -hide 113 \
 		-overview $(LOCAL_PATH)/core/java/overview.html