Using computer's proxy settings
Sharing settings.php isn't good practice at the best of times. 

Instead, it would be better to make that file specific to your own machine, and exclude it from version control. Have the rest of your dev team do the same. Then the problem is mostly solved, and you'll find it impossible to accidentally overwrite a production site's settings.php as a bonus.

Regarding automatically updating when you change your password - I think you're out of luck.

The way drupal_http_request() uses the proxy vars means you need to provide URL, username and password in plaintext as variables (i.e. the $conf array). I may be wrong, but I don't think you can get at those system credentials through PHP. Perhaps there's an extension that provides it, but I haven't come across one before. I'm almost positive core PHP doesn't have anything useful anyway.