Don't attach pipeline to group expansion mgr.

This is to avoid a bug where the group state is broken after a
notification is removed and then re-added (e.g. after being filtered by
zenmode), and ExpandableNotificationRow.areChildrenExpanded ends up out
of sync with ExpandableNotificationRow.isGroupExpanded.

Bug: 282865576
Test: tested manually that the bug doesn't happen anymore
Change-Id: Id3066a9269b9484a30f16bd43236b061b0b6aefe