[Notif redesign] Make group header respect header height

The parent of the group header was overriding the height of the header
to a different value from the one set in the view. Now that value
matches the height we expect for the redesign.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I5570c0805850096fa0825ca81a89f43278499e2b