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