Process input events immediately when received.

Reduce the latency for handling input events by ensuring that they are
handled as soon as they are received from the dispatcher.  This also
makes it easier for the input system to intelligently batch motion events.

This is part of a series of changes to improve input system pipelining.

Bug: 5963420
Change-Id: I0b3f6cbb3de5ac3c4eed35dfc774d6bd4a0b07fc