[Extensions + Apps] Move the arcAppsPrivate API to platform_apps

Move the arcAppsPrivate API out of extensions code and into apps
code. This includes:
- The API schema (arc_apps_private.idl)
- The API features (from _*_features.json files)
- The API permission (from c/b/e/permissions/chrome_api_permissions.cc)
- The API implementation and tests (from c/b/chromeos/extensions/)

Additionally, this updates the namespace for the API implementation to
be ::chrome_apps::api, for uniformity with the other apps API
namespaces.

Bug: 877240

Change-Id: Ie0ec379984eea3e02222d5877f34e916a225d4bd
Reviewed-on: https://chromium-review.googlesource.com/c/1298227
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Commit-Queue: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616667}
19 files changed