shill: cellular: Simplify CellularService name generation.

Before this CL, the name for cellular service was generated with help from the
cellular capabilities. After this CL, a default initial name is chosen for the
service, which is updated when the network operator has been determined.

BUG=chromium:352237
TEST=- Manual: Check service name shows correctly in UI for different
       operators.
     - Run shill unittests.
     - Run cellular_ServiceName.

Change-Id: I2a25ad076a645a1fe3d85023e2eb9df777ec089e
Reviewed-on: https://chromium-review.googlesource.com/193970
Reviewed-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>