[service-manager] Delete ServiceFactory interface

Removes the ServiceFactory interface from Service Manager's public API.
Packaged service creation is instead integrated into the main Service
interface with a new CreatePackagedServiceInstance message.

Bug: 925860
Change-Id: I66143f4483856fadc619accf54e279289acef471
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1593977
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#657888}