shill: cellular: Improve CellularCapability*::IsRegistered().

This CL makes CellularCapability*::IsRegistered() a const method and
adds a unit test for CellularCapabilityUniversal::IsRegistered().

BUG=None
TEST=Build and run unit tests.

Change-Id: I4171f2127822942c2633ac3479c4c5028bfda576
Reviewed-on: https://chromium-review.googlesource.com/176872
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/cellular_capability_universal_cdma.cc b/cellular_capability_universal_cdma.cc
index 0e5d612..f51892f 100644
--- a/cellular_capability_universal_cdma.cc
+++ b/cellular_capability_universal_cdma.cc
@@ -463,7 +463,7 @@
       (activation_state_ == MM_MODEM_CDMA_ACTIVATION_STATE_ACTIVATING);
 }
 
-bool CellularCapabilityUniversalCDMA::IsRegistered() {
+bool CellularCapabilityUniversalCDMA::IsRegistered() const {
   return (cdma_1x_registration_state_ !=
               MM_MODEM_CDMA_REGISTRATION_STATE_UNKNOWN ||
           cdma_evdo_registration_state_ !=