[Notif redesign] Update conversation icon sizes

Also removed a bunch of logic for updating paddings and sizes when
expanding, or based on whether it's a 1:1 conversation or a group, since
now the sizes are more consistent across the board. I may need to add
some of it back since currently the message icons overlap the face pile
in the expanded group conversation, but it's easier to start from a
clean slate.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: Ice6fa50ecaf6ab4442515d711af0714a8e11e0c2