Refactor InputQueue as InputEventReceiver.

This change simplifies the code associated with receiving input
events from input channels and makes it more robust.  It also
does a better job of ensuring that input events are properly
recycled (sometimes we dropped them on the floor).

This change also adds a sequence number to all events, which is
handy for determining whether we are looking at the same event or a
new one, particularly when events are recycled.

Change-Id: I4ebd88f73b5f77f3e150778cd550e7f91956aac2