Make bundle headers small all the time (whether expanded or collapsed).

This change adjusts the padding for the sub-components of the bundle header, and also adjusts the calculation of the header height within NotificationChildrenContainer to account for bundle headers being smaller than standard notifications.

Fixes: 435691562
Test: manual
Flag: com.android.systemui.notification_bundle_ui

Change-Id: If65b7d46dd56d0f97199fe5bfcb27759d2aa5dd1