[Flexiglass] Use WindowInsets when positioning the NSSL on DualShade

The floating DualShade panel is positioned inside the safeDrawing area,
which accounts for the screen corner radius, and the waterfall insets.

This logic was not mirrored in the bounds outputted by the
SharedNotificationContainerViewModel, which lead to some horizontal
misalignment on some screens.

Fixes: 417938924
Test: check the horizontal alignment of notifications in the floating
DualShade panel
Flag: com.android.systemui.scene_container

Change-Id: I6e67fa9d87fa3e1f2b77a2fad7da01ab408774b2