Moved NetworkChangeManager service definition to //services/network

Bug: 785420
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_mojo
Change-Id: I84bf1a42081c4890f8641a56a9cdb87404e25f67
Reviewed-on: https://chromium-review.googlesource.com/772938


Reviewed-by: Helen Li <xunjieli@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Randy Smith <rdsmith@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518291}