shill: cellular: simplify property change notification

Simplify property change notification by using a single mechanism to
handle it instead of two.  Old style notifications are turned into new
style notifications by the modem object.

A few minor style clean ups -- remove std:: where not needed and
ensure that public interfaces are declared as public interfaces in
subclasses (i.e not protected).

BUG=chromium-os:29569
TEST=run unit tests

Change-Id: If8b957d8b6518c95fa1d20bdef7d1ec6a8ed5ea3
Reviewed-on: https://gerrit.chromium.org/gerrit/20395
Tested-by: Jason Glasgow <jglasgow@chromium.org>
Reviewed-by: Nathan J. Williams <njw@chromium.org>
Commit-Ready: Jason Glasgow <jglasgow@chromium.org>
diff --git a/modem_1.cc b/modem_1.cc
index df71d67..af2c2da 100644
--- a/modem_1.cc
+++ b/modem_1.cc
@@ -109,4 +109,8 @@
   CreateDeviceFromModemProperties(modem_properties->second);
 }
 
+string Modem1::GetModemInterface(void) const {
+  return string(MM_DBUS_INTERFACE_MODEM);
+}
+
 }  // namespace shill