Make the throttling more sensitive to end-to-end latency.

The code now takes into account the time when the event was generated.
So with a 60Hz throttling rate, it ensures that a motion event will not
be delayed due to throttling by any more than 1/60th of second past
the time it was generated.

Change-Id: Iaea1d4f76e79036b4a18873485230731c3cd63c3