Disable PNG crunching for reproducibility

This can help to prevent non-determinism introduced by the crunching
process.

More information:
https://gitlab.com/fdroid/fdroid-website/commit/e48f9f0773568d92ff764ad3cc79eed0ae2c497e

With enabled and disabled crunching,
the size of the signed release APK was 17809681 bytes.

Related to #164