Adds documentation for a test echo endpoint service

Bug: 383574936
Change-Id: Ib631bd96de7b9ea09114d61dc5f6f3e89061d79a
Test: None, documentation only
diff --git a/contexthub/aidl/default/ContextHub.cpp b/contexthub/aidl/default/ContextHub.cpp
index 7eb51d0..80c9575 100644
--- a/contexthub/aidl/default/ContextHub.cpp
+++ b/contexthub/aidl/default/ContextHub.cpp
@@ -245,7 +245,7 @@
 
     Service echoService;
     echoService.format = Service::RpcFormat::CUSTOM;
-    echoService.serviceDescriptor = "ECHO";
+    echoService.serviceDescriptor = "android.hardware.contexthub.test.EchoService";
     echoService.majorVersion = 1;
     echoService.minorVersion = 0;