Hide notification stack footer even if the expansion is changing

Removed displaying of the footer (manage/clear all buttons) when
notification stack is expanding/collapsing and there is no space for it.
It makes sense to not display the footer while expanding/collapsing
as it won't be visible after finishing the motion anyway.

Bug: 187297685
Test: receive notifications so there is no space
    for the footer without scrolling,
    open the notification shade =>
    footer should not be visible during opening
Change-Id: Ic4fb55473ba9a4f6f04eac036295143789783557