TouchEvent (1/n): Adding TouchModeEvent to InputChannel

This CL detaches the touch mode state update from focus update. It does
that by introducing a new internal event (TouchModeEvent). This CL also
adds this event to InputChannel and related processing logic to
InputPublisher and InputConsumer. InputDispatcher will process two
different events now: FocusEvent when gaining/losing focus and
TouchModeEvent when entering/leaving touch mode.

Test: atest libinput_tests
Bug: 193718270
Change-Id: Ie4e5b6e8e798f12d7203127b4559fa40d38788de