[Notif redesign] Expander: always show number for groups

Previously, we were only showing the number when the group was
collapsed. In the redesign, we now want to show it all the time.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I6a3a7e72157d89ceda0a43432a8da2e6131fbf4d