[Flexiglass] offset NSSL drawbounds by its top margin

This is only causing issues if NSSL is not laid out to the top of the
window, but for correctness it should be offseted by the y position as
well.

Bug: 440059830
Test: check nssl_debug_lines
Flag: com.android.systemui.scene_container

Change-Id: I0d8e5210d279ef297e46346c86cf1397bfc737f3