[RN][iOS] Fix warning when loading RCTUIManager and A11yManager

[cipolleschi]

Summary:

When we fixed the race condition between A11yManager and RCTUIManager, we did it by moving the A11yManager on a background queue.
In the old architecture, this was raising a warning which our users might find confusing. Plus, that change was not aligned with what the A11yManager declared in its configuration because we are actually initializing it starting from a BG queue.

With this change we anticipate the initialization of the module in a place where:

1. We know we are in the main queue
2. We know we are going to need it (so it is not violating the lazy load principle)
3. We know it is safe.
   This should allow us to also remove the feature flag of RCTUIManagerDispatchAccessibilityManagerInitOntoMain because now it is safe to use the main_queue as requested by the module.

Changelog:

[iOS][Fixed] - Initialize the A11yManager in the main queue and when we need it.

Test Plan:

Tested in an app running 0.73.3, using the following configurations:

• Old Arch | Debug
• Old Arch | Release
• New Arch | Debug
• New Arch | Release

[github-actions]

	Warnings
⚠️	📋 Missing Summary - Can you add a Summary? To do so, add a "## Summary" section to your PR description. This is a good place to explain the motivation for making this change.
⚠️	Can't find the E2E test log for ci/circleci: test_e2e_ios. Job link

Generated by 🚫 dangerJS against c76fa44

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	8,499,851	-8,651,144
android	hermes	armeabi-v7a	n/a	--
android	hermes	x86	n/a	--
android	hermes	x86_64	n/a	--
android	jsc	arm64-v8a	9,754,834	-10,775,531
android	jsc	armeabi-v7a	n/a	--
android	jsc	x86	n/a	--
android	jsc	x86_64	n/a	--

Base commit: 4c108aa
Branch: main

[gabrieldonadel]

LGTM, Thanks for fixing this 👍

[jomavazquez]

Hi!
I still have the problem. Doing right now:
npx react-native@latest init MyProject --template react-native-template-typescript

which install RN 0.73.3, I get the error in the physical device and also in the simulator. I don't think it is related to RN-Reanimated library btw