ConnMan: use separate text entries for each proxy server type

As ConnMan allows us to store any arbitrary string value as a proxy server
URL we need to take steps in the UI to encourage the user to enter
appropriate values. This change adds separate entries for each proxy
protocol and uses placeholder text to demonstrate appropriate values.
ConnMan: add a protocol to each proxy server URL if not explicitly set

If the user enters a proxy server URL without protocol automatically add an
appropriate protocol to aide PACRunner et al in determining which server URL
is correct.