[scenic] Revert "[scenic] Convert ViewTokenPair to object"

This reverts commit 426805c5.

Reason for revert: ViewTokenPair is published in the Fuchsia SDK, so this change was a hard API transition for SDK consumers (e.g. Chromium & ANGLE).

Original change's description:
> [scenic] Convert ViewTokenPair to object
>
> This unifies the API for C++, Dart, and Rust better.  All 3 language
> bindings are pretty idiomatic now.
>
> Tested: view_token_pair_test
> SCN-1214 #comment
>
> Change-Id: I00bcbde5ff64c747ce427764cdfffdf6530e1295

TBR=abarth@google.com,jjosh@google.com,dworsham@google.com,jsankey@google.com,adamgousetis@google.com

Change-Id: Ie9219897b794e5adc38ad5e84a79ee99c1491bca
No-Presubmit: true
No-Tree-Checks: true
No-Try: true