[Notif redesign] Expander: highlight unopened groups

Highlight the expand button for groups that have never been expanded.

Note that we may also want to consider highlighting the pill when
there's an unseen notification in the group, but we can do that as a
follow-up.

There's also a KI where if a minimized group is expanded, then the shade is closed and re-opened, it gets re-inflated again with the highlighted button.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: Ib628f9546bf540020f71bc83bc8f4c413f30afe6