Fix SOCKS proxies from being dropped from PAC

It seems that SOCKS isn't being handled explicitly by the
PacProxySelector, which results in them just being dropped
from the return list.  This will sometimes end up switching
from SOCKS to DIRECT, which could be bad.

Bug: 17104885
Change-Id: Ic8a28230d3ae18c0abb000811a9100787c10c5e0