Hide silent notifications section header on lockscreen

Change things so we remove the gentle header when we're on the lock
screen. We still want to show the break between sections, so we
also modify beginsSection() to count either the header or the first
gentle child if there is no header.

Test: manual
Bug: 132689331
Change-Id: I02708a3a532cc2b9df2d1cec8192387dff132152