[App Management] Add App Management and App Service Ash flags.

This CL adds flags for the App Management and App Service Ash features,
to allow them to be enabled/disabled from the browser.

The App Management feature now also enables all the App Service
functionality used by the app management page, while the App Service Ash
feature enables the App Service functionality used by the shelf and app
list.

Bug: 906508
Change-Id: Ibd80ba73f50fc968bdd2ebbbd0e52cc893e870df
Reviewed-on: https://chromium-review.googlesource.com/c/1433332
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Nigel Tao <nigeltao@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: Reka Norman <rekanorman@google.com>
Cr-Commit-Position: refs/heads/master@{#627162}
11 files changed