InputDispatcher: Send cancellations from losing focus windows

An input channel must have at least one focusable window to gain focus.

When focus changes, we send cancellations for all of the non-pointer
event streams going to the previously focused window. Send these
cancellations to a window instead of directly to the channel so that
events are always associated with a window.

We track focus by token, and there can be more than one window for a
token/input channel. For such cases, we pick the first window (i.e. the
top-most window) with the focused token as the target, for convenience.

Bug: 210460522
Test: atest inputflinger_tests
Change-Id: I63990ccfca1f5ef013f6f08ce6336b4a961c20d6