Remove InputChannel from WindowState

We don't need to store the inputchannel inside the WindowState. When a
window is closed, the WM is already calling "removeInputChannel" into
the dispatcher, which would close the fd on the dispatcher side (destroy
the server-side channel), and thus notify the app that the channel is
closed. We don't need to dispose the InputChannel.

Long-term, we will attempt to remove the InputChannel::dispose
functionality.

Bug: 323450804
Test: opened apps and then relaunched them from recents
Flag: com.android.input.flags.remove_input_channel_from_windowstate
Change-Id: Id447bc987683e1bb580572010b21ae3e766e859c