MobileSetupUI: Avoid calls to shill when getting cellular info

Instead of getting cellular network service and device properties
directly from shill, use the values cached in chromeos::NetworkState and
chromeos::DeviceState.

The only piece of information that was not currently cached in
NetworkState was payment portal post data (data that should be sent to
the payment portal with POST requests) - added
chromeos::NetworkState::payment_post_data()

Along the way fix usages of CallJavascriptUnsafe and remove some
unneeded includes.

BUG=778021

Change-Id: Ie848686cc8d82154b40e4d944d83f503aac2e363
Reviewed-on: https://chromium-review.googlesource.com/c/1406190
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622228}
4 files changed