[Nearby] Add copy constructor to NearbyShareDecryptedPublicCertificate

Change-Id: I2e4fda0aae8c4f644c6e0ef79c410554611199cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2347976


Auto-Submit: Josh Nohle <nohle@chromium.org>
Commit-Queue: James Vecore <vecore@google.com>
Reviewed-by: James Vecore <vecore@google.com>
Cr-Commit-Position: refs/heads/master@{#796713}