Change session.remove to take an IBinder instead of an IWindow

Removing the window will tear down the input connection for an embedded
window. To make it more generic and allow non windows to have
InputChannel, it's simpler to pass in an IBinder instead of creating a
fake IWindow.

Test: WindowStateTests
Bug: 278757236
Change-Id: Ie632429e3c890077d6c4788941c686977d1ba5c9