transcoding: add watchdog to prevent transcoder hang

Add a watchdog to monitor transcoder progress. Make transcoder
report heart beat regularly as long as there is new progress.
If heartbeat stops, watchdog will initiate a timeout to

1) Abandon old TranscoderWrapper. We try to shut it down nicely,
however, if it's really stuck, we'll have to leave it there.
2) Instantiate a new TranscoderWrapper with new looper.
3) Report Watchdog timeout to client.

Tests:
- New unit tests to MediaTranscoder, TranscodingSessionController
and MediaTranscodingService's simulated test (for error code reporting).
- Manually tested that long recording works properly without timeout.

bug: 169453212
Change-Id: Iae89e49e8e12d6078dc49eef2960efd03e91c431