Make InputChannel private inside WindowState

The input channel token is already exposed, and in places where we are
accessing the channel we are actually after the token.

Make the channel private in this CL, and use the token instead where
needed.

Bug: 323450804
Test: atest android.view.surfacecontrol.cts.SurfaceControlViewHostTests
Flag: EXEMPT refactor
Change-Id: I9284f0bf7346384d0fb4f46b2f31ba3f5c1d2f9c