Fix UI API called on a background thread (#550, #530, #447)

[exxbrain]

Platforms affected

iOS

Motivation and Context

It fixes #530, fixes #550, fixes #447, fixes #509, fixes #515

Description

The plugin initializes a picker in the main thread now.
Previously made pull request by @ryaa: ryaa@a998ef4

Testing

I've launched my app and tried to start camera session. It is ok now. No exceptions or low speed.

Checklist

• I've run the tests to see all new and existing tests pass
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)

[breautek]

Thanks for the PR. Seems like there was a lot of confusion here for managing these tickets, between issues being closed prematurely even an existing PR was closed prematurely...

For github to recognise which issues this PR resolves, please edit it so it includes "fixes ", note that you need to repeat this format for each issue

fixes #issue
fixes #issue

See https://help.github.com/en/github/managing-your-work-on-github/closing-issues-using-keywords for more information.

Also I believe this PR will address #447 as well, please confirm for me.

[exxbrain]

Also I believe this PR will address #447 as well, please confirm for me.

Yes. Thank you. I've added it to the description as well.

[stevenleija]

When is this update expected to be released?

[gvsakhil]

Please merge these fixes as this camera delay opening is a real trouble making in may devices

[timbru31]

LGTM:

• Used demo project from First camera.getPicture() costs 10 or more seconds to open native camera #447
• Error message is fixed, Xcode no longer complains after moving the block
• Camera startup is no longer 10s

[AndrWeisR]

When is version 4.2.0 going to be released to npm? Why the delay?

[erisu]

4.2.0 is broken and failed release vote.