The easiest way to disable your app’s preview when your app is shown in task switcher is FLAG_SECURE.

When this is enabled, your app won’t display any previews, and instead shows a blank screen for most devices.

This can be done by creating a base Activity class that all activities extend, containing this in the onCreate:

protected void onCreate(@Nullable Bundle savedInstanceState) {
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_SECURE, WindowManager.LayoutParams.FLAG_SECURE);

Alternatively, you can selectively enable it on pause / resume (although the first approach is better):

override fun onPause() {
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_SECURE, WindowManager.LayoutParams.FLAG_SECURE)

override fun onResume() {

Originally from my (ultimately unhelpful) StackOverflow answer.