[Nearby] Add mojo type traits for ShareTarget

Also adds device type and name to the ShareTarget mojo struct to pass
them along to the UI. ShareTarget is converted to a struct as we need to
modify the fields when deserializing from mojo.

Bug: 1108367
Change-Id: I1e9e38cad39b6a3b9f4ac8de528c92ed1a06c3a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314356


Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Alex Chau <alexchau@chromium.org>
Commit-Queue: Richard Knoll <knollr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791468}