fix(window magnification): Set the surface of SurfaceView black

Since the mirrored surface might has an alpha less than 1, users will see the contents below the magnifier. Therefore, we would like to set the parent surface to black to provide a black background below the mirrored contents.

Test: atest WindowMagnificationControllerWindowlessMagnifierTest
Bug: 299981434
Flag: com.android.systemui.add_black_background_for_window_magnifier
Change-Id: I1fffab82b90ec97ea8d9d3cfb63e44e2a2681648