Show FSI Hun on lock screen in fully expanded shade

FSI HUN does not show up when shade is fully expanded on lockscreen. With this CL, we extend FSI Hun behavior for keyguard state.

To do that:
  - We need to prevent setting alpha of NSSL to 0. `updateNotificationTranslucency`
  - We need to set the initial position of  NotificationShelf to the end of the Stack, to fix unnecessary shelf animation issue.
   We don't need to position it in the middle of the screen and it's position is going to be managed by SSA after.
Bug: 277939036
Fixes: 271805874
Test: Manual. Incoming HUN is pinned to top of stack while notifications are scrolled (no regressions)
atest NotificationPanelViewControllerTest
atest NotificationShelfTest
atest StackScrollAlgorithmTest.kt

Change-Id: I35fd450aea9cdb863d04049cf60e6f8e94df7f4c