Upgrade zopfli to 831773bc28e318b91a3255fa12c9fcde1606058b am: a7aca0225b

Original change: https://android-review.googlesource.com/c/platform/external/zopfli/+/1736801

Change-Id: I8be4c6df965dad9b49c3e0eee521033d27709210