shill: cellular: Obtain |actvation_code| from serving operator.

In automatic activation implemented in CellularCapabilityUniversalCDMA, obtain
the activation code from the serving operator.

BUG=chromium:376098
TEST=(1) Run shill_unittests
     (2) Run network_CDMAActivate

Change-Id: Id56dcd99625ad2d655e58b68090a17febb7a56f9
Reviewed-on: https://chromium-review.googlesource.com/201084
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>