Check and follow this document to release the build by creating the Keystore and releasing the APK.
If you have an existing keystore, skip to the next step. If not, create one by running the following at the command line:
keytool -genkey -v -keystore ~/key.jks -keyalg RSA -keysize 2048 -validity 10000 -alias key
** $ flutter build apk --release
** $ flutter build --release