Keep the shade footer visible during clear-all, if there is a button to display

When we have non-clearable, and clearable items in the shade, and we
click the clear-all button.
We want to keep the footer visible, as there is still a button to
display.

Fixes: 295278931
Test: post a clearable and a non-clearable notification, press
clear-all, observe the animation
Test: atest StackScrollAlgorithmTest

Change-Id: I89611a03f077c36cf7e0cf5c516d256a2afde395