[Notif redesign] Top-align top line in expanded notifs

In expanded notifications, the top line of text showing the app name,
timestamp etc should be aligned with the top of the icon (so 16dp under
the top of the notification, which is the margin used for most things in
the redesign).

Also align the content that comes below it accordingly.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I93d624ad4cd46fd454286c8771ca6bb9d15c74ac