chromeos-dbus-bindings: Generate only single proxy accessor for singletons

For D-Bus objects with single fixed object path assume that they are
singletons and remove the object path parameter from the proxy
accessor ObjectManagerProxy::GetNNNProxy() method.

BUG=brillo:348
TEST=`FEATURES=test emerge-link chromeos-dbus-bindings`
     `FEATURES=test emerge-link peerd privetd buffet apmanager lorgnette`

Change-Id: I29419e2db5bd0fc8a4f508804c1e322c1880a3dd
Reviewed-on: https://chromium-review.googlesource.com/252422
Trybot-Ready: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Alex Vakulenko <avakulenko@chromium.org>
Reviewed-by: Christopher Wiley <wiley@chromium.org>
Commit-Queue: Alex Vakulenko <avakulenko@chromium.org>
diff --git a/buffet/buffet_client.cc b/buffet/buffet_client.cc
index 5a8a72a..e630dc8 100644
--- a/buffet/buffet_client.cc
+++ b/buffet/buffet_client.cc
@@ -146,12 +146,11 @@
       return return_code;
 
     object_manager_.reset(new org::chromium::Buffet::ObjectManagerProxy{bus_});
-    auto manager_instances = object_manager_->GetManagerInstances();
-    if (manager_instances.empty()) {
+    manager_proxy_ = object_manager_->GetManagerProxy();
+    if (!manager_proxy_) {
       fprintf(stderr, "Buffet daemon was offline.");
       return EX_UNAVAILABLE;
     }
-    manager_proxy_ = manager_instances.front();
 
     auto args = CommandLine::ForCurrentProcess()->GetArgs();