Remove elevation from SystemUIDialog.

Elevation is now unused and creates issues due to a weird
interaction with Window Manager. The proper fix would be in Window
Manager, but this is a band-aid on our end to avoid issues.

Bug: 404508609
Flag: com.android.systemui.spatial_model_app_pushback
Test: manually tested that the flicker is gone when launching from dialogs
Change-Id: Ie45efff9d440070dff15ed648ca89b0a212ba2c8