[Fast Pair] Add API to repository to get saved devices and opt in status

Adds an API to the FastPairRepository to get a list of saved devices and
the user's opt in status from Footprints. This differs from the API
added in earlier CL's to get just the opt in status since this will be
used by the Saved Devices page to populate the settings page, whereas
the call that just checks the opt in status is used before we write
an account key.

TEST=unit tests

Change-Id: Ib4c45fcdd508e377412c4a7c01a1ef3b86c1f83b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3532824
Reviewed-by: Shane Fitzpatrick <shanefitz@google.com>
Commit-Queue: Juliet Lévesque <julietlevesque@google.com>
Cr-Commit-Position: refs/heads/main@{#983502}