[Notif redesign] Bigger "small" icon in header only notif

Note that notification_2025_template_header is a fork of
notification_temmplate_header.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I0d65856b0ea550cf20aa4be08b07a00dfaa01e53