[Flexiglass] Correct NSSL#drawBounds for the horizontal padding of the NSSL

The received bounds from the placeholder are relative to the window, but NSSL.setDrawBounds
expects a rect that's relativet to the NSSL itself. This CL offsets these bounds in case the
NSSL has some horizontal margins e.g. in floating DualShade.

Bug: 417965077
Test: check the box drawn by the debug lines in all shade modes
Flag: com.android.systemui.scene_container
Change-Id: I925513e56c95ebe3a3c08bddf6ea34ffcb6e8317