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This is part of a series moving IO on evdev devices to a fast thread,
since UI thread is too slow for the cursor.

This splits EventFactoryOzone into two parts: EventFactoryOzone and
InputDeviceFactoryOzone. The new InputDeviceFactoryOzone class is now the
device I/O-centric part, and is intended to exist on a low-latency thread.

Hotplug events still originate on the UI thread, but this is not a
problem because hotplug is not latency critical. We're sharing udev
with the display code, so its easiest leave that code alone for now.

This doesn't add a new thread yet. Everything still runs on the UI thread.

BUG=449710
TEST=boot link_freon & move mouse

Review URL: https://codereview.chromium.org/863353003

Cr-Commit-Position: refs/heads/master@{#313629}