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This change adds discovery session support to FakeCentral in order to
allow Bluetooth scanning to be simulated.

BUG=719826,719827

Change-Id: Ia5150cc14924e9e03e5361797068db6718b26595
Reviewed-on: https://chromium-review.googlesource.com/954231Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542349}