EXO: Refactor wl_shm interface

Move the wl_shm interface into its own files. This change only moves
code around, no functional changes.

Bug: 896710
Test: Build and ran exo_unittests
Change-Id: If553a9db14ea9bc965f28667a685f60d5446735a
Reviewed-on: https://chromium-review.googlesource.com/c/1412692
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Commit-Queue: Daniel Nicoara <dnicoara@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623724}
4 files changed