Make DBus a singleton.

All the proxies need dbus to initialize, so we are passing dbus around,
to make it easier to compile dbus conditionally, this patch makes it a
singleton so that the proxies can get dbus on their own.

Test: mma
Bug: 28800946

Change-Id: Idf062c843aa34a431c2201bae5b895dc1d0ea787
diff --git a/Android.mk b/Android.mk
index 141b144..78cdbdf 100644
--- a/Android.mk
+++ b/Android.mk
@@ -310,6 +310,7 @@
     connection_manager.cc \
     connection_utils.cc \
     daemon.cc \
+    dbus_connection.cc \
     dbus_service.cc \
     hardware_android.cc \
     image_properties_android.cc \