fuchsia: Fix renderer destructor

[arbreng]

#27423 introduced a regression even in the non-flatland code because of a member/destruction-order issue. This crash occurs when a flutter app is terminated and the engine instance is shutting down.

While debugging that, I discovered another underlying issue -- all render objects like FuchsiaExternalViewEmbedder are created on raster but destroyed on platform! This is very problematic, especially re: FIDL bindings which must only be accessed on one thread (the destructor logic sends FIDL commands).

Fix both issues.
@uysalere

Tests: Manual testing with workstation (launch simple_browser then quit) and smart_display (session_ctl restart)
Fixes: https://bugs.fuchsia.dev/p/fuchsia/issues/detail?id=83473

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[chandarrengoog]

Nice fix! should we add a test for this in the future?

[arbreng]

Nice fix! should we add a test for this in the future?

Yes, https://bugs.fuchsia.dev/p/fuchsia/issues/detail?id=83736 will address this in the future