Use correct focus token for SCVH#getFocusGrantToken

SCVH#getFocusGrantToken currently returns the root focus grant token for
WindowlessWindowManager. This isn't correct since the WWM could have
created multiple windows. Instead, use the window token from SCVH to
request the specific focusGrantToken that's stored in a map in WWM.

Fixes: 277197084
Test: SurfaceControlViewHostTests
Change-Id: If01cd0b165d574963964bd4f28b6e21b87f9478f