Flutter homescreen volume control not working

With the changes already pushed for SPEC-4762, Flutter applications like the Flutter homescreen need to be switched to using the KUKSA.val databroker gRPC API to ensure their use of VSS signals still works.  This is visible in the homescreen with the main volume control not working, as it is updating the older KUKSA.val server via its WebSocket API while the agl-service-audiomixer daemon is listening to the databroker exclusively now.  Since this user-facing behavior is the most visible breakage from the interim state of the switch to the databroker, and has caught out when running the manual test suite, explicitly track it with this issue.