[Nearby Presence] Update SharedCredential and add it to PresenceDevice

This CL updates mojom::SharedCredential to match SharedCredential in
//third_party/nearby/src/internal/proto/credential.proto and introduces
it into mojom::PresenceDevice in order to stay aligned with
//third_party/nearby/src/presence/presence_device.h.

A follow-up CL will update mojom::LocalCredential and corresponding
conversions between LocalCredential <> SharedCredential.

Test: updated unit tests
Fixed: b/328450930
Change-Id: Iee5a7ad7f1dee8af8126153fc034cb74c6dd7cdd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5366350


Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Reviewed-by: Juliet Lévesque <julietlevesque@google.com>
Reviewed-by: Emily Stark <estark@chromium.org>
Commit-Queue: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282720}