[fuchsia] Update to new FIDL API
[fuchsia] Update to new FIDL API

There's an incoming change to use std:: types for non-nullable strings
and vectors.

See: https://fuchsia-review.googlesource.com/c/garnet/+/236996

Change-Id: Ib924e812a679213928a26dbf29402f40cb5f4cb3
Reviewed-on: https://dart-review.googlesource.com/c/88122
Commit-Queue: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>

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