InputDispatcher: Refactor focus logic into FocusResolver

Pull out logic that keeps track of the focused window per display and
the logic used to determine focus changes to a new class. Input
Dispatcher will feed in focus requests & window handle changes. It
will then consume any focus changes.

There is no functional changes in this cl. Additional logging has been
added when we drop a focus request.

Test: atest inputflinger_tests

Change-Id: I04c0c38907cca5d6d98405a6a9f7eb7c497fd90f