EXO: Refactored zcr_notification_shell interface into its own file

EXO: Moved zcr_notification_shell_v1_interface into another file,
to be followed by refactoring of more protocols.

Bug: 896710
Test: Compiled on ChromeOS.
Change-Id: I0884132357f8acadb8d5659b9330abae8acc7cd1
Reviewed-on: https://chromium-review.googlesource.com/c/1302861
Commit-Queue: Shawn Gallea <sagallea@google.com>
Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607424}
4 files changed