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Adds the abstract manager classes for contacts, certificates, and local
device data, whose eventual implementations will direct data flows to
and from the Nearby server and local storage.

Bug: b/154865027, b/154863644, b/154863722
Change-Id: I3a6947541b3167a144b7c6524850b1ec8a7887b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2272121
Commit-Queue: Josh Nohle <nohle@chromium.org>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#789746}