How to disable your app's preview image in Android's task / app switcher

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);
    super.onCreate(savedInstanceState);
}

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

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

override fun onResume() {
    super.onResume()
    getWindow().clearFlags(WindowManager.LayoutParams.FLAG_SECURE)
}

Originally from my (ultimately unhelpful) StackOverflow answer.