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After connecting to Cable authenticator using advertisement/scanning,
Cable protocol requires one more round trip of BLE control messages to
be sent to establish shared secret key for encryption. Add this
handshake protocol to FidoCableDiscovery.

Bug: 849307
Change-Id: I388fa43dae60e97e027dafa1f9f430a2569ca981
Reviewed-on: https://chromium-review.googlesource.com/1089794
Commit-Queue: Jun Choi <hongjunchoi@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568334}