Do not set dispose callback when flag is enabled

When input channel is disposed, it triggers the dispose callback, which
will explicitly remove the channel from the dispatcher, even if there
are other open fds for this connection on the client side.

This behaviour is different from the finalizer. When the input channel's
finalizer runs, it will delete the native object, but will not trigger
the dispose callback.

Therefore, with the new flag, we should avoid setting the dispose
callback on the input channel. This way, when the flag is rolled out, we
can directly remove the dispose callback functionality.

Bug: Bug: 323450804
Test: opened apps and then relaunched them from recents
Flag: com.android.input.flags.remove_input_channel_from_windowstate
Change-Id: I5a2566c41cb7c6650f3e84f8430b29fcac8ddef2