[Notif redesign] Make text flow below expander in collapsed notifs

Note that mHeadingFullMarginSet was unused, so I repurposed it for this.

Fix: 403536710
Test: screenshot tests
Flag: android.app.notifications_redesign_templates
Reland: I77256ebd9d2c9ba5cfe7a2c51606b6ecc775c6cb
Change-Id: I17e6a805bb74bd4ad9ee9259ff6793ba5246570d