[Notif redesign] Update spacing of collapsed group children

This is to match the redesign specs, including aligning to the new,
larger "small" icon.

Note that notification_2025_hybrid and
notification_2025_hybrid_conversation are forks of hybrid_notification
and hybrid_conversation_notification respectively.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I18ded714d191f7dcc082806f0b4984c29e66b9de