HiddenApiKey.gradle

//In local.properties
tmdb_api_key = "xxxxxxxxxxxxxxxxxxxxxxxxxx"

//In build.gradle (Module: app)
buildTypes.each {
        Properties properties = new Properties()
        properties.load(project.rootProject.file("local.properties").newDataInputStream())
        def tmdbApiKey = properties.getProperty("tmdb_api_key", "")

        it.buildConfigField 'String', "TMDB_API_KEY", tmdbApiKey
        
        it.resValue 'string', "api_key", tmdbApiKey

}

//In your Constants File
var tmdbApiKey = BuildConfig.TMDB_API_KEY