Begin Migrate NSSL

When Flags.migrate_nssl is enabled, move NSSL to be a sibling of both
keyguard_root_view and notification_panel. More work is needed for
touch handling before moving to teamfood.

Also, there were 2 files testing
NotificationsQSContainerController... consolidated them, and then
split them to a "legacy" one where the new flag is off, and the normal
one where the flag is true.

Bug: 287644451
Fixes: 288237173
Test: atest NotificationStackScrollLayoutControllerTest
NotificationsQSContainerControllerTest
SharedNotificationContainerInteractorTest SharedNotificationContainerViewModelTest

Change-Id: I291f460d6d5cfd95e338394accca37802b19bbfe