h265decoder: Fix for unhandled low-delay decoding case

Baseclass calls get_preferred_output_delay() in a chain of sequence header parsing and new_sequence() call. Thus latency query should happen before the codec data handling. This commit is to fix wrong liveness signalling in case of upstream packetized format.

The same change as !2363 (merged) but separate MRs because we don't need to backport this