[CrOS MultiDevice] Move ProximityAuth WebUI code to MultiDevice.

The MultiDevice component is the correct location for shared
multi-device code, including the chrome://proximity-auth debug UI.

Bug: 912743
Change-Id: I788ce0eba9528159b1859cb45303f0816fc19959
Reviewed-on: https://chromium-review.googlesource.com/c/1383114
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Rahul Chaturvedi <rkc@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617666}
26 files changed