Add API 'since' tag for O dev preview (current.txt)

Change-Id: I10ffd161f9249794df7d263a11703fbca0d88451
diff --git a/Android.mk b/Android.mk
index b204da7..109f610 100644
--- a/Android.mk
+++ b/Android.mk
@@ -931,6 +931,7 @@
     -since $(SRC_API_DIR)/23.txt 23 \
     -since $(SRC_API_DIR)/24.txt 24 \
     -since $(SRC_API_DIR)/25.txt 25 \
+    -since ./frameworks/base/api/current.txt O \
 		-werror -hide 111 -hide 113 \
 		-overview $(LOCAL_PATH)/core/java/overview.html