Replace sun.net.www.ParseUtil#decode with public API

We currently use a static helper method, sun.net.www.ParseUtil#decode, to decode a 'file:' URL on Windows before trying to open a connection (see RT-22458). The sun.net.www package is an internal JDK pacakge. We need to remove the dependency on this internal API and instead use public API to parse the URL in this case (possibly copying from the internal class if there isn't a suitable replacement.