[Notif redesign] Change color of message icon bg

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

Bug: 406224300
Fix: 410732505
Test: manual
Flag: android.app.notifications_redesign_templates
Change-Id: I45119c15e5a5fa228cb04e5f342816f2b78d3495