Ensure input events are processed in-order in the application.

As it turns out, it used to be possible for there to be multiple
input events simultaneously in flight in an application.  Although
it worked, it made it hard to reason about what was going on.
The problem was somewhat exacerbated by the introduction of a
queue of "InputEventMessage" objects as part of an earlier latency
optimization.

This change restores order from chaos and greatly simplifies the
invariants related to input event dispatch within the application.

Change-Id: I6de5fe61c1fe2ac3dd33edf770d949044df8a019