Fix bug where lockscreen notification is minimized and not dismissable

This happened because
- recently we started counting MediaHeaderView as part of
  maxDisplayedNotifications on lockscreen
- causing the last notification that should have shown on lockscreen
  to restrict itself to shelf height, and later be replaced
  with the shelf during layout; the shelf is not dismissable.

The fix is to
- ignore MediaHeaderView when counting shown notifications
- still include MediaHeaderView in contentHeight calculations

Fixes: 187304623
Test: have MediaHeaderView and many notifications on lockscreen
      and swipe away all of them individually
Change-Id: I4e122b9321856880a5a09e03df61698eaf6b5f54