Fix RNDIS: start host iface properly, update device default gateway.

Fix RNDIS: override default gateway and leave network manager running.

BUG=417894, 417889
Committed: https://crrev.com/f2d01c1247ccb364793dc5a807ac8af0c5973ec7
Cr-Commit-Position: refs/heads/master@{#297553}

[slamm, 2014-09-26 00:25:26]

slamm@chromium.org changed reviewers:
+ tonyg@chromium.org, wuhu@google.com

[slamm, 2014-09-26 00:25:26]

RNDIS is broken on latest Google-issued Ubuntu hosts.

My change makes default gateway override happen all the time. (Hubert said this
should work.)
The change also drops the code to stop the network-manager since it is critical
for the network interfaces to work. (I unsure how we could detect what older
systems might need to do that.)

Tony, do the bots need that network manager stopping code?

[slamm, 2014-09-30 22:13:33]

Ping, tonyg.

[tonyg, 2014-09-30 22:15:39]

lgtm

[slamm_google, 2014-09-30 22:23:17]

The CQ bit was checked by slamm@google.com

[commit-bot: I haz the power, 2014-09-30 22:24:21]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/605873002/40001

[commit-bot: I haz the power, 2014-10-01 00:05:28]

Committed patchset #3 (id:40001) as 4491b0053985ab5000303a6f720beed559dd39bc

[commit-bot: I haz the power, 2014-10-01 00:06:18]

Patchset 3 (id:??) landed as
https://crrev.com/f2d01c1247ccb364793dc5a807ac8af0c5973ec7
Cr-Commit-Position: refs/heads/master@{#297553}