Set TaskSnapshotWindow no InputChannel.

TaskSnapshotWindow has no need to process any input events.
If it has a valid InputChannel but no InputEventReceiver,
it won't receive events sent by InputDispatcher like TouchModeEvent.
This may cause an ANR.

Test: when TaskSnapshotWindow is showing, verify that touch on
Notification shade won't trigger ANR on TaskSnapshotWindow.

Bug: 271552817

Change-Id: I9595b15fa277ebff4be375eed220cb57533a1a2c