splitmuxsink: Only set running time on finalizing sink element when in async-finalize mode

There is only a single sink element in async-finalize mode, and we would keep the running time from previous fragments set in that case. As we don't ever set the running time for the very last fragment on EOS, this would mean that the closing time reported for the very last fragment is the same as the closing time of the previous fragment.

CC @thaytan