shill: Handle setting of cellular's AllowRoaming property.

Disconnect the cellular device when roaming and roaming is disallowed. Broadcast
changes in the property.

BUG=chromium-os:25849
TEST=unit tests

Change-Id: I099f85152871cf8e64e96ded72b40712bd66dbd1
Reviewed-on: https://gerrit.chromium.org/gerrit/15406
Tested-by: Darin Petkov <petkov@chromium.org>
Reviewed-by: Eric Shienbrood <ers@chromium.org>
Commit-Ready: Darin Petkov <petkov@chromium.org>
diff --git a/cellular.h b/cellular.h
index 578f32c..705b794 100644
--- a/cellular.h
+++ b/cellular.h
@@ -184,6 +184,7 @@
   FRIEND_TEST(CellularTest, StartCDMARegister);
   FRIEND_TEST(CellularTest, StartGSMRegister);
   FRIEND_TEST(CellularTest, StartLinked);
+  FRIEND_TEST(CellularCapabilityTest, AllowRoaming);
   FRIEND_TEST(CellularCapabilityTest, EnableModemFail);
   FRIEND_TEST(CellularCapabilityTest, EnableModemSucceed);
   FRIEND_TEST(CellularCapabilityTest, GetModemInfo);