[Media] Make play/pause button always reserve space.

This helps the cards all have the same height even if some of them don't
show the play/pause button.

Bug: 397989775
Test: manually verified the height of the cards is uniform in the
compose gallery app
Flag: EXEMPT not using this yet

Change-Id: Iddfedd07b1a52e23149807e8f2f026e73227c535