fix(share_plus): on ios use top view controller instead of root

[evgeny-sureev]

Description

There was an error in iOS implementation of share_plus plugin.
In case of sharing texts or files, instead of top view controller, root view controller was passed.
Sharing of Uri uses correct view controller.

Error appears only in scenario where existing iOS app uses Flutter module, which in turn uses share_plus.

Related Issues

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.