Update to libpng 1.6.19

Removes all Android modifications, including
SK_PNG_INDEX_SUPPORTED (for subset decoding).

BUG:23265085
BUG:25424175
Change-Id: Iaa4f6cc9fc6db8e4b46a83211363adfcbf07b589
diff --git a/tests/pngunknown-vpAg b/tests/pngunknown-vpAg
index 396c6e4..04f07b6 100755
--- a/tests/pngunknown-vpAg
+++ b/tests/pngunknown-vpAg
@@ -1,2 +1,2 @@
 #!/bin/sh
-exec ./pngunknown vpAg=if-safe "${srcdir}/pngtest.png"
+exec ./pngunknown --strict vpAg=if-safe "${srcdir}/pngtest.png"