Ensure init is called on CommunalHostViewController

This change make sure NotificationPanelViewController calls
init on CommunalHostViewController after it is constructed.
It also moves callback registration in CommunalHostViewController
to onViewAttached, as that will be called by init.

Bug: 195132590
Test: atest NotificationPanelViewTest#testCommunalhostViewControllerInit
Change-Id: Ia4163413b576e5c344f14a6ec025aae8e9128b58