face: move isListening()/size() to Channel as pure virtual methods

As a side effect, this ensures that Channel cannot be instantiated.

Change-Id: I875876ddf8fc0fbb04f3e88726c708ece8106702
diff --git a/tests/daemon/mgmt/face-manager.t.cpp b/tests/daemon/mgmt/face-manager.t.cpp
index 8856b0a..41dcb90 100644
--- a/tests/daemon/mgmt/face-manager.t.cpp
+++ b/tests/daemon/mgmt/face-manager.t.cpp
@@ -247,6 +247,18 @@
   {
     setUri(FaceUri(uri));
   }
+
+  bool
+  isListening() const final
+  {
+    return false;
+  }
+
+  size_t
+  size() const final
+  {
+    return 0;
+  }
 };
 
 class TestProtocolFactory : public face::ProtocolFactory