shill: Use service_constant's kTypeCellular to create storage id.

BUG=chromium-os:24952
TEST=unit tests

Change-Id: I1428887187ab5ec4cc44389997ccc5aac0bfe2a6
Reviewed-on: https://gerrit.chromium.org/gerrit/15226
Commit-Ready: Darin Petkov <petkov@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/cellular_capability_gsm.cc b/cellular_capability_gsm.cc
index cf4e551..7e61185 100644
--- a/cellular_capability_gsm.cc
+++ b/cellular_capability_gsm.cc
@@ -139,7 +139,8 @@
   // If IMSI is available, base the service's storage identifier on it.
   if (!imsi_.empty()) {
     cellular()->service()->SetStorageIdentifier(
-        "cellular_" + cellular()->address() + "_" + imsi_);
+        string(flimflam::kTypeCellular) + "_" +
+        cellular()->address() + "_" + imsi_);
   }
   cellular()->service()->SetActivationState(
       flimflam::kActivationStateActivated);