[Notif redesign] Change color of icon bg in hybrid conversations

We missed this place where the bg was being used when we updated it in I7caf1c930c708fc048175a2864ece1665f81475a.

Bug: 406224300
Test: manual
Flag: android.app.notifications_redesign_templates
Change-Id: I839762c7247b0db703c7605633af3e2889234b73