Re-lands #109232 with a fix for google_mobile_ads

There was a subtle race in the previous version of the PR, which google_mobile_ads hit but the plugin I was testing with did not: because _sendCreateMessage is async, even if it returned false synchronously due to null size, create didn't necessarily continue synchronously. This created a brief async gap for TLHC where the state was creating, and thus awaitingCreation would return false, and the second create call from PlatformViewLink wouldn't fire.

This fixes that (see second commit in the PR) by using a synchronous getter rather than an async bool return value from the subclass to know whether to drop null-size calls to create.

Fixes (at least part of) #107297

Pre-launch Checklist

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