Snippet to custom keystore file for release apk file.

android-custom-release-keystore.gradle

/**
 * Create your key store: keytool -genkey -v -keystore release-key.keystore -alias alias_name -keyalg RSA -keysize 2048 -validity 10000
 * and after update your build.gradle file.
 */
 android {
    signingConfigs {
        release {
            storeFile file("release.keystore")
            storePassword "******"
            keyAlias "******"
            keyPassword "******"
        }
    }
    buildTypes {
        release {
            signingConfig signingConfigs.release
        }
    }
}