build-config.gradle

def propertiesFile = project.rootProject.file('local.properties')
if (propertiesFile.exists()) {
    Properties properties = new Properties()
    properties.load(propertiesFile.newDataInputStream())

    ext.SECRET_KEY = properties.getProperty('SECRET_KEY') ?: ""
} else {
    ext.SECRET_KEY = System.getenv('SECRET_KEY') ?: ""
}

build.gradle

...

android {
    defaultConfig {
        ...

        resValue 'string', 'application_id', applicationId
        resValue 'string', 'version_name', versionName
        resValue 'string', 'secret_key', SECRET_KEY
    }
}