InputDispatcher_test: Remove ability to create clone windows via c'tor

There were previously two ways to create clone windows: using the
FakeWindowHandle's clone() method, or by passing in an existing token
into the FakeWindowHandle c'tor.

Refactor the FakeWindowHandle c'tor so that the only way to create a
cloned window is using the clone() method. There should be no behavior
change.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I07270feff6c32481b7faaa070eb1c511e1399989