Cherry-pick from master Doc change: Add API 26 to @since list

Add API 26 to @since list and turn off preview watermarking.
Fix has been successfully submitted in master and affects doc files
only.

Bug: 62087404
Bug: 64123587
Test: make ds-docs

Original change-Id: Idcf84882a1a1fe0864ee6ff3d0af6cd857c73a06

Change-Id: I28700ad15d931256d9dfc1016df546bd2f0e718f
diff --git a/Android.mk b/Android.mk
index 3f2ae83..3f059ae 100644
--- a/Android.mk
+++ b/Android.mk
@@ -950,7 +950,7 @@
     -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 \
+    -since $(SRC_API_DIR)/26.txt 26 \
     -werror -hide 111 -hide 113 -hide 121 \
     -overview $(LOCAL_PATH)/core/java/overview.html \