[App Management] Implement the App Service Uninstall method for extensions.

This CL implements the Uninstall method for the ExtensionApps App
Service publisher, and links the UI to this method through the
App Service, allowing extensions to be uninstalled in response to UI
events.

Bug: 906508
Change-Id: I7108e5b3d6f684b80fe996b720c75a7af3a318bc
Reviewed-on: https://chromium-review.googlesource.com/c/1405150
Reviewed-by: calamity <calamity@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Nigel Tao <nigeltao@chromium.org>
Commit-Queue: Reka Norman <rekanorman@google.com>
Cr-Commit-Position: refs/heads/master@{#622376}
16 files changed