[shill] FindService should return a ServiceRefPtr, and should always return something

We weren't explicitly returning a value from FindService on failure,
which was causing a unit test to pass erroneously.  Once I fixed it,
the test started failing because of a dumb thing I was doing in
MockService.  So, fix that so that the tests can pass as they should.

BUG=None
TEST=unit tests

Change-Id: Ice304e514ea0e3add779ac061b0945b5712dea29
Reviewed-on: http://gerrit.chromium.org/gerrit/2374
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Paul Stewart <pstew@chromium.org>
diff --git a/manager.h b/manager.h
index 8b6ac60..2ad0d72 100644
--- a/manager.h
+++ b/manager.h
@@ -40,7 +40,7 @@
   void FilterByTechnology(Device::Technology tech,
                           std::vector<DeviceRefPtr> *found);
 
-  Service* FindService(const std::string& name);
+  ServiceRefPtr FindService(const std::string& name);
 
  private:
   scoped_ptr<ManagerAdaptorInterface> adaptor_;