Proximity Auth: Remove kMultiDeviceApi flagging from ProximityMonitorImpl.

R=hansberry@chromium.org

Bug: 899324
Test: none
Change-Id: I331612216739e82fb46d674aef5853b7777b7a7d
Reviewed-on: https://chromium-review.googlesource.com/c/1325549
Commit-Queue: James Hawkins <jhawkins@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606564}
4 files changed