InputDispatcher: Ensure synthesized events are associated with windows

For tracing purposes, we want as much information as possible about
where input events are dispatched. Ideally, we will trace the windowId
that is the target of each dispatched event. That way, we can easily
look up properties of the window that received the event.

The difficulty with this is that there is a many-to-one mappings between
windows and input channels, where more than one window can receive input
through the same channel. A channel might also receive input when there
no windows associated with it - such as if a window has been removed and
we want to generate cancelation events for it, or such as for global
monitors that don't have windows at all.

We make the best effort to associate each event dispatch with a
windowId. Then in the tests, we consume events from the window that we
expect to be associated with the event.

For cloned windows that are both touched at the same time, the entire
gesture is usually associated with the window that was first touched,
because the pointers from both windows will be merged into the same
event.

For focus events, we track focus by token and not by the window, so when
there is more than one window for a focused token, we arbitrarily pick
the top-most window to associate events with for tracing.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I47744cbd677cc74e26a102c50a2c11c68bc8aa89