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This implementation:
- Stores the set of selected contacts to prefs, but the full contact
  list must be downloaded as needed.
- Handles the upload of contacts to the Nearby Share server.
- Schedules periodic check-ins with the server to see if contacts have
  changed since the last upload.

Bug: b/154863722, b/154861871
Change-Id: I72c03ea0f922f3386fd6ea5336fbab728f0e429c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2307477Reviewed-by: James Vecore <vecore@google.com>
Commit-Queue: Josh Nohle <nohle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801720}