WindowManager: Communicate with input system by WindowTokens.

Once we switch to a model of passing InputWindowInfo through
SurfaceFlinger, the handles will be parcelled and we will
no longer receive the same literal object back from "interceptKeyBeforeQueueing"
or other input callbacks. This means the approach of saddling a WindowState
on to the object for later retreival will no longer work. Instead we pass
the IBinder WindowState#mClient in when registering the input channel, and modify the
input system to pass it back to us as a sort of UUID.

Bug: 80101428
Bug: 113136004
Bug: 111440400
Test: Home button still works.
Change-Id: I0f41e0d08b034aa037518c3a8fb21be1453565da