Set correct value of MultilineTextInput.readonly for iOS.

[o-fedorov]

On iOS, MultilineTextInput is read-only by default and when readonly is False, and editable when readonly is True.

Fixes #1859

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[o-fedorov]

I would appreciate if you suggest how to add tests for this change.

[freakboy3742]

@o-fedorov This is an area where our docs are a bit slim, so I've just added a PR (#1868) to add details on how the testbed app and backend tests work. You can see a preview of these docs here.

At present, there aren't any backend tests for MultilineTextInput, so any improvement here will be a win. If those docs don't make any sense, or you can't work out how to build a test for readonly, let us know - we can merge what you've got here so you're unstuck on the specific bug. We're in the process of auditing all the widgets, so we should be adding tests fairly soon.

[o-fedorov]

@freakboy3742 thank you for the response. I have created a copy of this PR with tests added: #1869

Feel free to do anything with these PRs, like close this one and update the other PR.

[freakboy3742]

Thanks - I'll close this in favour of #1869, since that looks pretty close to being merge ready.