[Notif redesign] Align expander vertically in one-line notifs

We already align it for header-only notifications, but collapsed
(headerless) notifications that don't have a text look similar and
didn't have the same centering behavior.

Fix: 398222413
Bug: 378660052
Test: tested manually, including updating the same notification
with/without a text to make sure the expander comes back to the right
place
Flag: android.app.notifications_redesign_templates

Change-Id: I86e50b4df03a94d45865c47c41b21663d78e77b3