Upgrade zopfli to 831773bc28e318b91a3255fa12c9fcde1606058b am: a7aca0225b am: eb419370f4 am: 23e37b1188 am: a361033957

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

Change-Id: I71ee4039f553ead0f25e849bf1cfb189152064d1