Fix toString() for empty DeviceIdSet

This makes parsing more consistent.

Bug: 367816690
Change-Id: I818aea519d14cdf34d3beeacc02b723034408265
Test: make
Flag: android.media.audio.routed_device_ids
diff --git a/media/libaudiofoundation/AudioContainers.cpp b/media/libaudiofoundation/AudioContainers.cpp
index 6727562..7964e6d 100644
--- a/media/libaudiofoundation/AudioContainers.cpp
+++ b/media/libaudiofoundation/AudioContainers.cpp
@@ -132,7 +132,7 @@
 
 std::string toString(const DeviceIdSet& deviceIds) {
     if (deviceIds.empty()) {
-        return "Empty device ids";
+        return "AUDIO_PORT_HANDLE_NONE";
     }
     std::stringstream ss;
     for (auto it = deviceIds.begin(); it != deviceIds.end(); ++it) {