[Status Bar Refactor] Move PanelBar#collapsePanel into

NotificationPanelViewController.

PanelBar#collapsePanel mostly just called through to
PanelViewController, so the function might as well belong in PVC.

Test: Manually verified that the same functions are called in the
correct order in various scenarios (open shade, close shade, open QS
tile into new activity, close QS via home button while in app)
Test: atest SystemUITests
Bug: 200063118

Change-Id: I3c5d41db7daf8085baea73db806c6e350a7115bf