[Notif redesign] Replace notification_content_margin_end

Replacing the old margin with the general notification_2025_margin used
for most things in the redesign. They have the same 16dp value so this
doesn't actually change anything visually.

Also updated some leftover usages of the old notification_content_margin
in the reply history container.

Bug: 378660052
Test: screenshot tests should be unchanged
Flag: android.app.notifications_redesign_templates
Change-Id: I173a9a8baa480c445009af12884a9dd869cb259c