Made sure that the media section has no background

The background is unnecessary and UX wise doesn't look
good when scrolling. We therefore remove it from
the media header.
Because the screen animation still depends on having the
bounds calculated correctly, we now make all of the
background views ExpandableViews instead of
ActivatableNotificationRows.

Bug: 154137987
Test: add media notification, observe no background, normal screen on / off animation
Change-Id: Ie18af9d61d8d128c7a9397660910403e87c6fd51