shill: cellular: Unify UpdateStorageIdentifier.

Before this CL, |CellularService::SetStorageIdentifier| was called from
different code paths in three of the capabilities. This CL unifies these code
paths to be triggered when the ServingOperator is determined in |Cellular|.

BUG=chromium:352243
TEST=- Manually test that sensible storage identifiers are used for different
       operators, and that they are persistent across service reboots.
     - Run shill unit-tests.

Change-Id: I5837465589667e4d4feb75eeeeeb6f793fb5450a
Reviewed-on: https://chromium-review.googlesource.com/197602
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>