Fix small buttons in collapsed layout

Updates the end barrier to be constrained by the large play/pause button
rather than parent. This fixes the small buttons not being visible in
the collapsed layout if the large play/pause button is not shown.

Test: manual - verify buttons for both layout types in LTR and RTL
Fixes: 262562516
Change-Id: Ib1d72aefd89dfc7b977a743959e40db7359010fa