Upgrade zopfli to 831773bc28e318b91a3255fa12c9fcde1606058b am: a7aca0225b am: eb419370f4

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

Change-Id: I79824eb3fbf10820d9b5e66d01b7d05c0b224ec2