Support for treating single party IMS conference as a standalone call.

Adding @hide APIs which Telephony can use to make a conference call with
a single participant look like its a standalone call.

Test: Manual testing
Bug: 75975913
Change-Id: Id8532234ab295785fc749b120898f43911e12637

telecomm/java/android/telecom/RemoteConnectionService.java

@@ -471,6 +471,12 @@ final class RemoteConnectionService {
         public void resetConnectionTime(String callId, Session.Info sessionInfo) {
             // Do nothing
         }
+
+        @Override
+        public void setConferenceState(String callId, boolean isConference,
+                Session.Info sessionInfo) {
+            // Do nothing
+        }
     };
 
     private final ConnectionServiceAdapterServant mServant =