Unregister input channel by token

An input channel is uniquely identified by its token. Once the input
channel exists, you can unregister it simply using its token.

There's no need to pass back the entire input channel, including its id,
to input dispatcher. We will look for the server-side input channel
using the token anyways.

Bug: 167947395
Test: atest libinput_tests
Change-Id: I009da372bd6cc897b019a66e9c3bbed78cc91223