[Notif redesign] Increase top margin of expanded messaging content

This ensures that the icons in the content don't overlap the icon in the
header. It also makes spacing similar to conversation notifications.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I71085d947aaf9deccb29e554df2f8f583a7ca9e5