[AOD RONs] Fix header centering logic

When updateHeader is called on a "expanded" notification, the titleView was null (because the title is not in the header). This tricked the singleLine logic into triggering even though there was a title on the second line.

This only triggered if there was no text in the notification.

This CL also clarified the naming of various params in order to add clarity about this oddness that the title can be in multiple different places.

Fix: 421938041
Test: checked redacted and unredacted, both with and without text
FLag: android.app.ui_rich_ongoing
Change-Id: I905c2c70e1a43b9a6f4db32bb3b8568240b05535