Ensure each SCVH child window has a unique focus token.

Currently, all windows created via WindowlessWindowManager for the same
SCVH will have the same focus token. This will break if there are child
windows that also get focus since WMS will not have a way to uniquely
identify the tokens.

Change-Id: I3c3de947d2e03f3a0b82aa3dde374c3851a64ca5
Test: SurfaceControlViewHostTests
Bug: 230340812