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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.

The keyboard layout is a UI thread object, so move all of the keyboard
processing that needs layout information outside of the device-specific
processing. Devices no longer get a KeyboardEvdev* since they will
execute on a different thread.

BUG=449710
TEST=boot link_freon & press key

Review URL: https://codereview.chromium.org/872883003

Cr-Commit-Position: refs/heads/master@{#313397}