Fix cursor misplacement problem when text-align is center or right.

[DeMonkeyCoder]

fixed issue flutter/flutter#53057

[DeMonkeyCoder]

I couldn't find documentation for running test files on the engine.
I think it would be better if you add it to your contribution guide.

[DeMonkeyCoder]

@GaryQian @justinmc

[justinmc]

I tried running this locally and it improves the situation, but I still see a problem:

To reproduce:

1. Create a multiline input with textAlign: TextAlign.right.
2. Type some text on the first line.
3. Hit return to go to the second line and type some text there.
4. Press the up arrow.
5. Type more text. The cursor appears on the left but text is inserted on the right.

[jason-simmons]

Thank you for the PR, and sorry for taking so long to review it.

I looked at this bug and posted a PR with a solution that I believe will work for all text alignment modes. See #17744

[DeMonkeyCoder]

Thanks for your review.
So I'm closing this one.