Further cleanup around DBus interface classes.

* Removed the "Glib" part from the various DbusGlib class name and added
  "Wrapper" instead: only part of the methods here declared have
  anything to do with Glib; in essence, this is an interface containing
  wrappers for various DBus functions.

* Changed "Dbus" to "DBus", to comply with the rest of the world.

* Renamed the actual implementation "RealDBusWrapper" (instead of
  "Concrete").

* Separated out RealDBusWrapper into its own header file. Client code,
  which often only cares about the abstract class, does not need to pull
  it unless specifically requiring it.

* Cleaned up the includes in these headers.

Aside from all that, also renamed system_state.cc into
real_system_state.cc, as this is what this file contains.

BUG=None
TEST=Unit tests.

Change-Id: I015c407cbc159aba8b5925eb0e916ba604c829cd
Reviewed-on: https://chromium-review.googlesource.com/189373
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
diff --git a/main.cc b/main.cc
index 345d6e1..9098063 100644
--- a/main.cc
+++ b/main.cc
@@ -19,8 +19,8 @@
 
 #include "update_engine/certificate_checker.h"
 #include "update_engine/dbus_constants.h"
-#include "update_engine/dbus_interface.h"
 #include "update_engine/dbus_service.h"
+#include "update_engine/dbus_wrapper_interface.h"
 #include "update_engine/real_system_state.h"
 #include "update_engine/subprocess.h"
 #include "update_engine/terminator.h"