[Notif redesign] Bigger "small" icon in HUN

Note that notification_2025_template_heads_up is a fork of
notification_template_material_heads_up.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I32d3a355900cb10470600b3c467caa28e87d30da