Add @TestApi annotations to CS-side RTT APIs for CTS

CTS tests need both the in-call and connection service sides of the APIs
to test Telecom functionality properly, so annotating the hidden-for-now
connection service APIs with TestApi.

Test: part of CTS
Change-Id: I3711729d7e8c8aff2735f4da9fbd04bcca6b4942

telecomm/java/android/telecom/ConnectionService.java

@@ -1603,6 +1603,7 @@ public abstract class ConnectionService extends Service {
         Log.d(this, "stopRtt(%s)", callId);
         if (mConnectionById.containsKey(callId)) {
             findConnectionForAction(callId, "stopRtt").onStopRtt();
+            findConnectionForAction(callId, "stopRtt").unsetRttProperty();
         } else if (mConferenceById.containsKey(callId)) {
             Log.w(this, "stopRtt called on a conference.");
         }