Fix the shade footer alignment, if there is only one button

When there is only the history/manage button is visible in the footer,
it should be aligned to the start of the layout. This Cl fixes this alignment.

Fixes: 296044544
Test: post clearable and non-clearable notifications, and observe the
clear-all animation

Change-Id: I98d74f868e42cad03257c580373fe91a0d7d5cef