[Notif redesign] Adjust margins of "small" icon

The icons also don't need to be centered vertically with this new
margin.

There's a KI where the bottom of the icon gets cut off a bit when
expanding a group, but that will be fixed in a follow-up.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I173c765696067d3f2d0b7df87f31c713c0ebfe26