[Notif redesign] Consolidate MessagingGroup

The icons and alignment for the old messaging style and the new
conversation style are no longer different, so we don't need to override
things in MessagingGroup based on whether it's in a conversation or not.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: Ic25829bcc8120762621065fab37a10808eb2e7ec