Create a controller for NSSL

This is the first step towards removing view injection from
NotificationStackScrollLayout.

For right now, this doesn't seem to do too much. This CL passes the
controller into most places the view used to go, and that in turn
simply proxies requests through to the view.

A follow up CL will work to actually remove injection from
NotificationStackScrollLayout.

Bug: 149989572
Test: atest SytemUITests && manual
Change-Id: Ic161c36f090479fb1ccbfdd735a2a9ab5616f290