App login with FirebaseUI - launch UI flow

applogin.java

findViewById<Button>(R.id.btn_sign_in).setOnClickListener {
    startActivityForResult(
        AuthUI.getInstance()
            .createSignInIntentBuilder()
            .setAvailableProviders(listOf(AuthUI.IdpConfig.GoogleBuilder().build()))
            .build(),
        RC_SIGN_IN
    )
}