Decoder: add support for KEEP_THREADS_ACTIVE

Current design of Mpeg2 Decoder creates new threads
to process each frame. This CL adds support to reuse
the threads across frames.

Test: mpeg2dec -c test.cfg

Bug: 188027522

Change-Id: Iab1a75d8cfcddec065ca9b89fa314e544ab7608a
7 files changed