EXO: Refactor wl_data_device_manager interface

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

Bug: 896710
Test: Build and run exo_unittests
Change-Id: I3dcd3849e9e7766a97c0792dc17a2fbe84b0f312
Reviewed-on: https://chromium-review.googlesource.com/c/1435625
Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626564}
4 files changed