Previously, we were comparing the signed int `target_length` (returned by WideCharToMultiByte) to a size_t string length, resulting in a signed/unsigned comparison warning as follows:

```
windows\runner\utils.cpp(54,43): warning C4018:  '>': signed/unsigned mismatch
```

WideCharToMultiByte returns:
* 0 on error
* the number of bytes written to the buffer pointed to by its fifth parameter, lpMultiByteStr, on success.

As a result it's safe to store the return value in an unsigned int, which eliminates the warning.

No changes to tests since this is dependent on end-user project settings/modifications and does not trigger a warning with default project settings.

Fixes: https://github.com/flutter/flutter/issues/134227