Here is how to production build for Android:
cd android &&
./gradlew assembleRelease &&
cd app/build/outputs/apk/release &&
jarsigner -keystore YOUR_KEYSTORE_PATH -storepass YOUR_KEYSTORE_PASS app-release-unsigned.apk YOUR_KEYSTORE_ALIAS &&
zipalign 4 app-release-unsigned.apk app-release.apk
This will generate app-release.apk which should be good to go the play store (see android/app/build/outputs/apk/release folder).
Here is the jarsigner command manual.