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/shill_proxy.h b/shill_proxy.h
index 6d545f6..78f5b9b 100644
--- a/shill_proxy.h
+++ b/shill_proxy.h
@@ -32,7 +32,7 @@
 // This class implements the connection to shill using real DBus calls.
 class ShillProxy : public ShillProxyInterface {
  public:
-  explicit ShillProxy(const scoped_refptr<dbus::Bus>& bus);
+  ShillProxy() = default;
   ~ShillProxy() override = default;
 
   // Initializes the ShillProxy instance creating the manager proxy from the