Why background drawable retain its state after activity finished?

Question

I have a custom ViewGroup, It does only one thing, changing background's alpha on layout phase:

@Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
        ...
        getBackground().setAlpha(0);
        ...
}

print alpha in setBackground:

@Override
public void setBackground(Drawable background) {
    if (background instanceof ColorDrawable) {           
        bgAlpha = Color.alpha(((ColorDrawable) background).getColor());
        d("bg:" + Integer.toHexString(bgAlpha));
        super.setBackground(background);
    } else {
       ...
    }
}

Now I have a activity inflate this custom view from layout xml:

   <CustomViewGroup
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#88000000" />

When I first launch the app, setBackground print:

bg:88

then I finish the app and reopen it from launcher, setBackground print:

bg:0

Why background drawable retain its state after activity finished?