[Notif redesign] Animate top line in group header expansion

This change introduces a gradual transition when dragging to expand a
group, so the top line doesn't just jump to its centered position
when the expansion is done.

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: I21ef172ba5ad14404e1c863c209a35bada3790d4