[Notif redesign] Animate expander alignment in group header expansion

This change introduces a gradual transition when dragging to expand a
group, so the expand button doesn't just jump to its centered
position when the expansion is done. The arrow itself still only
switches positions once the transition is finished.

The on-click transition is not affected (i.e. it's still jumpy).

Bug: 378660052
Test: visual test
Flag: android.app.notifications_redesign_templates
Change-Id: I4a444111f3b3eaa57ce50015ef28694dc70090cf