Position NSSL by scene

Keyguard and the shade will each need to position the
NSSL (SharedNotificationContainer) based upon which scene is
active. The KeyguardInteractor will position it when on keyguard, and
NPVC will position it when the shade is being pulled down and
active. Migrate this logic out of NPVC and into the
SharedNotificationContainerViewModel.

The work isn't yet complete. Additional bugs have been filed to
continue the cleanup.

Bug: 288237177
Test: atest SystemUITests

Change-Id: Idce5a4f8dd55c68b40d52bb7f5cbc7e5d360bb07