EXO: Refactored keyboard extension interface

Move keyboard extension out of server.cc
This CL only moves code around.

Bug: 896710
Test: Built on ChromeOS
Change-Id: I15ea4ac7f9c5cabf9470d0478cd91a409c0b5e6a
Reviewed-on: https://chromium-review.googlesource.com/c/1320028
Commit-Queue: Shawn Gallea <sagallea@google.com>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608439}
4 files changed