Update mHeadingFullMarginSet

This is currently unused, but we'll need it in this new format for two
follow-up fixes. Making this change in its own commit so that the other
two don't need to depend on each other.

Bug: 403536710
Bug: 407819104
Test: tested together with follow-ups, but it builds on its own
Flag: EXEMPT unused code (for now)
Change-Id: I7a056c8548854ca786a57e0dfc09d7f12e64031a