add signing using keystore file to an android build.gradle file

build.txt

android {
    compileSdkVersion ...
    .....

    // Note this section MUST appear before 'buildTypes' in the build.gradle file.
    signingConfigs {
        release {
            storeFile file("filename.keystore")
            storePassword "password"
            keyAlias "alias"
            keyPassword "password"
        }
    }

    buildTypes {
        release {
            ....
            signingConfig signingConfigs.release
        }
    }
    
    ...
}