DO NOT MERGE Update libpng to 1.6.20 BUG:23265085 Change-Id: I85199805636d771f3597b691b63bc0bf46084833 (cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82) (cherry picked from commit 2642b28cdaa24f8f9df12807a2dc90caafc8f407)