[Notif redesign] Update conversation layout

We're now using two separate layouts for the collapsed and expanded
version of conversations, like we do for all other styles. The structure
is also more consistent with the other templates.

CallLayout temporarily uses the old conversation header so that it
doesn't crash, it will be updated as well in a follow-up CL.

Files notification_2025_template_collapsed_conversation.xml and
notification_2025_template_expanded_conversation.xml are based on
notification_2025_template_collapsed_messaging.xml and
notification_2025_template_expanded_messaging.xml respectively.

The new notification_2025_conversation_header.xml is based on
notification_2025_header.xml.

Bug: 378660052
Test: tested manually, screenshot tests to come later
Flag: android.app.notifications_redesign_templates

Change-Id: Ib21bae19aee26a77f7d7f4fd4812df64d804bdde