Reenable SocksProxyTest

ojluni SocksSocketImpl supports SOCKS v5 sufficiently for these tests to pass.

I will close https://code.google.com/p/android/issues/detail?id=96926 as
FutureRelease once this change is merged.

Tested:

  cts-tradefed run cts -c com.squareup.okhttp.SocksProxyTest

Change-Id: I19c458bf6713b1cd23d6d2fea7187e2faf6a6664