Change Input to use IBinder intead of IWindow for client token.

There's no need for Input to use IWindow object since it's not use the
interface and just using it as a pure client token. Change the system
server code so it accepts an IBinder object instead of IWindow so more
generic tokens can be used to create InputChannels.

Test: Input works
Bug: 278757236
Change-Id: I8eaa0832a9b70818a94ee3e21b7f3269456af9a2