[Notif redesign] Allow space for entire header in expanded groups

When a group is expanded, the header should be visible at its full
height. Adjust the dimensions used in NotificationChildrenContainer
accordingly to achieve that.

I also renamed some of these fields to make it more clear what they
actually represent:
- mNotificationHeaderMargin is now mCollapsedHeaderMargin
- mNotificationTopPadding is now mAdditionalExpandedHeaderMargin

Bug: 378660052
Test: expand groups both by tapping and dragging; screenshot tests tbd
Flag: android.app.notifications_redesign_templates
Change-Id: If3ef79484a98311da4ee5404f610142255cb5dde