[flexiglass] fix shared element transition for Notification Placeholder in Lockscreen
During Lockscreen -> Shade/Split Shade transition, we move the NotificationPlaceholder to be a sibling of the scrim so that the two can move independently. Also adds a new shared element key that should be used for this specific transition.
Bug: 330484293
Bug: 330466650
Test: manually verified NotificationPlaceholder transitions smoothly during Lockscreen -> {Split}Shade
Flag: com.android.systemui.scene_container
Change-Id: I31f7e492134cfdfad151669be7e41c853badb80f
4 files changed