CollpaseActivity.kt

class CollapseActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_collapse)
        setSupportActionBar(toolbar)

        //Seta o título na actionbar e depois muda a cor para transparente quando o AppBar estiver expandido
        collapsing_toolbar_layout.title = "Ronaldo Fenômeno"
        collapsing_toolbar_layout.setExpandedTitleColor(resources.getColor(android.R.color.transparent))

        ...
    }
}