Add getHostToken API in AttachedSurfaceControl

Adds getHostToken API to the AttachedSurfaceControl. This API will
return the token to associate the host and embedded windows.

Bug: 304508760
Test: atest AttachedSurfaceControlTest#testGetHostToken
      atest ViewRootImplTest#whenViewIsAttachedToWindow_getHostToken
Change-Id: I8a45264eb70068012061daeb32dc1f56cd24905d