h264decoder: Fix for unhandled low-delay decoding case

Baseclass calls get_preferred_output_delay() in a chain of sequence header parsing and then new_sequence() is called with required DPB size (includes render-delay) information. Thus latency query should happen before the sequence header parsing for subclass to report required render-delay accordingly via get_preferred_output_delay() method. (e.g., zero delay in case of live pipeline)

This commit is to fix wrong liveness signalling in case of upstream packetized format.