[Notif redesign] Remove space under header when hiding sender

If the name of the conversation is also the name of the sender in the
top message, we hide the sender. In this case, let's also remove the
16dp space between the list of messages and the title.

We only do this for conversations (where we know the sender icon is in
the header), but not for legacy messaging notifications where we need
the extra space to display the icon underneath the app icon in the
header.

Bug: 399055060
Bug: 378660052
Test: manual + screenshot tests
Flag: android.app.notifications_redesign_templates
Change-Id: I10683febbc4d08ca175860a6cf0d0313ce5946f6