Fix cancel event sent to wrong back callback

When a callback is registered while a back gesture is in progress,
WindowOnBackInvokedDispatcher continues to dispatch the back events
to the active callback. It does so even when the newly registered
callback ends up being the new top callback in the callback stack.

A few code places in WindowOnBackInvokedDispatcher assumed that an
active gesture must always be dispatching to the top callback. However,
as the previous paragraph explains, that's not always the case. This CL
fixes this edge case problem.

Bug: 420650888
Test: WindowOnBackInvokedDispatcher
Flag: com.android.window.flags.predictive_back_callback_cancellation_fix
Change-Id: Iee98ee2222394ec4805c6e689347796771b636eb