[flexiglass] Always position NSSL at the y=0 in the window

* This removes the ViewPosition model because we never care about a top offset
* We still use the left offset to ensure that the clipping is correctly offset in split shade
* Logs a warning if the NSSL y-position is ever not at the top
* Disables y-translation of the NSSL for burn-in
* Adds support for burn-in translation of the notification placeholder.
  * Note: while this will shift the placeholders in X, the only X we use from the placeholder is the scrim, which isn't part of the lockscreen, so it doesn't matter.

Fixes: 332750091
Test: atest SystemUITest
Test: atest SystemUiRoboTest
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Flag: ACONFIG com.android.systemui.migrate_clocks_to_blueprint TEAMFOOD
Change-Id: I9cbe228cb50b449e0c5d6dfee902c322e6ce4b7a