transcoder: Add startThreadPool to benchmark

Adding ProcessState startThreadPool fixes a
hang, related to Extractor, seen for running
benchmark for many assets or multiple
iterations.

Bug: 175889103
Test: ./MediaTranscoderBenchmark
--benchmark_filter=BM_TranscodeVideoPassthrough
--benchmark_min_time=60

Change-Id: Ib4f1bbf131b4e97b0cf3391747dc5541f5f8fcd9