Use correct heads-up notification height.

Before this change we would calculate heads-up notification height from
the notification private layout - which is incorrect in cases where the
private and the public layouts differ (e.g. when we have smart
suggestions in the notification showing in the private layout but not
the public one).

Bug: 128894557
Test: manual as per the bug.
Change-Id: I58d2b988402abcefe9c23aa74dd4f89ad953aed1