utils: streamproducer: update appsrc latency upon appsink latency event

Currently, the streamproducer sets the latency on the appsrc consumers once. If there is a latency change (e.g. from a network src in a upstream pipeline), such change is not propagated downstream to all the intersrc elements, causing the pipelines not to work correctly.

Proposed changes

• Set the consumers.latency_updated property to true whenever there is a Latency event on the producer's appsink.