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Uses SecureChannelClient to keep a connection open with the appropriate
remote device. This change also requires slightly tweaking HostScannerOperation
to delay unregistering the remote device in question until ConnectionPreserver
has fully had a chance to communicate with the Mojo SecureChannel service.

Bug: 824568, 752273, 855813
Change-Id: I6c8c915ab1efbae9c77e08c9c9b2406a5b4de2ad
Reviewed-on: https://chromium-review.googlesource.com/1117558
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570979}