shill: cellular: Reorganize default implementations in capability code.

This CL cleans up the cellular capability code as follows:
- Reorganizes the default implementations of method in the cellular
  capability classes
- Removes duplicated override methods
- Reorders method declarations in header files to group related methods
  together
- Adds the override keyword to override methods and fixes mismatched
  method signatures

BUG=None
TEST=Build and run unit tests.

Change-Id: If4adcd5852fc56cd472620385fdd0c5b358d0b11
Reviewed-on: https://chromium-review.googlesource.com/185048
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.cc b/cellular_capability_universal.cc
index 7374eb0..21dfa50 100644
--- a/cellular_capability_universal.cc
+++ b/cellular_capability_universal.cc
@@ -152,6 +152,8 @@
       &CellularCapabilityUniversal::SimLockStatusToProperty);
 }
 
+CellularCapabilityUniversal::~CellularCapabilityUniversal() {}
+
 KeyValueStore CellularCapabilityUniversal::SimLockStatusToProperty(
     Error */*error*/) {
   KeyValueStore status;
@@ -396,16 +398,6 @@
   }
 }
 
-void CellularCapabilityUniversal::DisconnectCleanup() {
-  SLOG(Cellular, 2) << __func__;
-}
-
-void CellularCapabilityUniversal::Activate(const string &carrier,
-                                           Error *error,
-                                           const ResultCallback &callback) {
-  OnUnsupportedOperation(__func__, error);
-}
-
 void CellularCapabilityUniversal::OnActivationWaitForRegisterTimeout() {
   SLOG(Cellular, 2) << __func__;
   const string &sim_identifier = cellular()->sim_identifier();