InputDispatcher: Optimize count()

Count items as they are added and removed rather than iterating over the
entire list to count them.

Increases performance slightly, particularly when tracing is turned on,
which causes count to be queried more often, and when the number of
items in the queues grow large.  This can happen due to applications not
responding, for example.

Change-Id: I0f11f7edd46089612af910cdfabfeb3ee685d7d9