transcoding: add thermal status listener

Register thermal status listener with thermal manager, and
pause/resume transcoding activity when thermal status goes
above certain threshold (currently set at CRITICAL).

Add unit tests with mock thermal policy. Also add unit test
that exercise the actual transcoding service, but use adb
shell cmd to fake thermal status change.

bug: 169452730
test: unit testing added in the CL; manual testing using
"adb shell cmd thermalservice override-status #" and observe
thermal throttling logs from transcoding service.

Change-Id: I7d28ed3069bdd73e8984ad138e2edba9c9a2360a