flimflam: Fix set_apn issues. This CL fixes set_apn to work on 64-bit platforms and be compatible with shill. BUG=chromium-os:33179 TEST=Tested the following commands with flimflam and shill: 1. set_apn 2. set_apn -c 3. set_apn -u <username> -p <password> <apn> 4. set_apn -n <network_id> -u <username> -p <password> <apn> Change-Id: I1a442d59e601586fee36a17d5daeea2919a24487 Reviewed-on: https://gerrit.chromium.org/gerrit/29195 Reviewed-by: Thieu Le <[email protected]> Commit-Ready: Ben Chan <[email protected]> Tested-by: Ben Chan <[email protected]>