I am sign apk for test on FGL site, and then using next commands in terminal (just replace path and file names to yours):
/Applications/android-sdk-macosx/build-tools/25.0.2/zipalign -f -v 4 /Users/alexeygulev/Downloads/andr/Bring+me+Cakes-enhanced.apk /Users/alexeygulev/Downloads/andr/aligment.apk
/Applications/android-sdk-macosx/build-tools/25.0.2/apksigner sign --key key.pk8 --cert certificate.pem /Users/alexeygulev/Downloads/andr/aligment.apk