Add flag for removal of inputchannel from windowstate

This will help clean up the InputChannel lifecycle. Eventually, we could
switch to only having InputChannel in the app process, which would allow
us to return an InputReceiver instead.

Flag: com.android.input.flags.remove_input_channel_from_windowstate
Bug: 323450804
Test: launch an app and then reopen from recents
Change-Id: I6d1789078b8a63ef774a2f4c1113d69ae2245fe0