[Notif redesign] Fix alignment in collapsed old messaging notif

I wanted to avoid adding more logic that's different between
conversations and old messaging style, but unfortunately without
rethinking how conversations are laid out it doesn't quite work. The
same messaging_group view already has a margin from the parent in the
collapsed messaging style, but not in conversations.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: Ic9f4919e959e90302a13005c4846989c4e035997