a11y: TextInput with IconButton makes textField unreachable via screenreader

Internal: b/140829496

Possibly shares a root cause issue #38940 (https://github.com/flutter/flutter/issues/38940#issuecomment-523567392)

If you have a textField with a suffix icon button
```
TextField(
  decoration:InputDecoration(
    suffixIcon: IconButton(...),
  ),
)
```

Then TalkBack is able to focus only on the icon button, and there's no way to activate the text field.