Simply approach to detech keyboard visibility changed on Xamarin.Android, based on blog of PDFKit (https://pspdfkit.com/blog/2016/keyboard-handling-on-android/#)

keyboard_helper_activity.cs

private void AddKeyboardVisibilityHandler()
{
    var decorView = Window.DecorView;
    decorView.ViewTreeObserver.GlobalLayout += OnViewTreeObserverGlobalLayoutChange;
}

private void RemoveKeyboardVisibilityHandler()
{
    var decorView = Window.DecorView;
    decorView.ViewTreeObserver.GlobalLayout -= OnViewTreeObserverGlobalLayoutChange;
}

void OnViewTreeObserverGlobalLayoutChange(object sender, EventArgs e)
{
    int MinKeyboardHeightPx = 150;
    var decorView = Window.DecorView;
    decorView.GetWindowVisibleDisplayFrame(_windowVisibleDisplayFrame);
    int visibleDecorViewHeight = _windowVisibleDisplayFrame.Height();

    // Decide whether keyboard is visible from changing decor view height.
    if (_lastVisibleDecorViewHeight != 0)
    {
        if (_lastVisibleDecorViewHeight > visibleDecorViewHeight + MinKeyboardHeightPx)
        {
            // Calculate current keyboard height (this includes also navigation bar height when in fullscreen mode).
            int currentKeyboardHeight = decorView.Height - _windowVisibleDisplayFrame.Bottom;
            // Notify listener about keyboard being shown.
            OnKeyboardShown(currentKeyboardHeight);
        }
        else if (_lastVisibleDecorViewHeight + MinKeyboardHeightPx < visibleDecorViewHeight)
        {
            // Notify listener about keyboard being hidden.
            OnKeyboardHidden();
        }
    }

    _lastVisibleDecorViewHeight = visibleDecorViewHeight;
}