TouchEvent (3/n) Enhancing InputDispatcher to support TouchModeEvent

This CL provides the infrastructure to dispatch TouchModeEvent via
InputDispatcher. Remaining logic like permission check, Java integration, etc will be added in next incoming CLs.

Note:
InputDispatcher::requestTouchModeChange instantiate and enqueue TouchModeEvent.
InputDispatcher::dispatchTouchModeChangeLocked notifies all windows
about the touch mode update.

Test: atest inputflinger_tests
Bug: 193718270
Change-Id: I3cfeec0e9bc33737f15d6822242f7eb5e381a119