EXO: Refactored wl_seat interface for cast usage

Move wl_seat interface out of server.cc,
removed usage of keyboard and pointer for cast shell.

Bug: 896710
Test: Built on ChromeOS
Change-Id: I3e91381883ff028e684b5c4bfbb1dd744cf31ed8
Reviewed-on: https://chromium-review.googlesource.com/c/1324099
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@{#608933}
4 files changed