DO NOT MERGE Update libpng to 1.6.20

BUG:23265085
Change-Id: I85199805636d771f3597b691b63bc0bf46084833
(cherry picked from commit bbe98b40cda082024b669fa508931042eed18f82)
diff --git a/tests/pngvalid-progressive-interlace-standard b/tests/pngvalid-progressive-interlace-standard
index 59042eb..a8cdf9e 100755
--- a/tests/pngvalid-progressive-interlace-standard
+++ b/tests/pngvalid-progressive-interlace-standard
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngvalid --standard --progressive-read --interlace
+exec ./pngvalid --strict --standard --progressive-read --interlace