Add some extra padding at the bottom of QS

This gives some room when QS is scrollable to scroll to the bottom of
media without it collapsing. This is not visible when QS is not
scrollable, as it's just part of the bottom empty space.

Test: manual, add media and scroll
Fixes: 412319163
Fixes: 394013533
Flag: com.android.systemui.qs_ui_refactor_compose_fragment
Change-Id: I554b3760e049a394d7764014c110f4c1c4f02840